Biggest reason being is that the boundaries are way too muddled.
While 1946-1964 may be the most concrete definition for Baby Boomers, it isn't universal, there are some definitions that start the Boomers in 1943, as well as some that end it in 1960, with Generation X starting in 1961, and some even split the Baby Boomers in half, into Post-War Boomers and Generation Jones. The X-Y and Y-Z boundaries are muddled as hell. You have some sources that end Generation X as early as 1976, and others ending it as late as 1984. The Y-Z boundary is by far the worst offender, you have Statistics Canada ending Generation Y as early as 1992, and then you have Strauss & Howe ending it as late as 2004, leaving a 12-year overlap!
That's why I gave this a lot of thought, and came up with a solution to this problem; a new generation theory, where each cohort spans 9-years. It's much much better than the current BB-X-Y-Z model if you ask me, because the dates aren't muddled, and the spans aren't too long, their coming of age years (16-20) should roughly consist of the same time period, in some way or another.
Here are the cohorts;
1883-1891 = Edwardian Generation
They are named the Edwardian Generation because they came of age in the 1900s decade, which is when King Edward VII reigned as King of the British Empire. Last generation to grow up as kids in the 19th Century and first to come of age in the 20th Century, they'd have more value for technologies such as cars and radio than future generations as those were just becoming mainstream by the time they were already adults.
1892-1900 = World War I Generation
Would've been coming of age or still in their early twenties during World War I, and therefore would've been the cohort most affected by it as they were in the perfect shape to fight in the war. Grew up as kids and teens during the 1900s and early 1910s when modern technologies such as automobiles and motion pictures were becoming mainstream, so they had a rather naive worldview growing up of how perfect the world was, until that worldview was destroyed by World War I in the mid 1910s.
1901-1909 = Prohibition Generation
All under 18 by the time World War I ended, and would've been coming of age in the prohibition era following that war. Grew up as kids and teens during World War I, which helped them develop a conscious worldview. Would popularize the jazz and movie culture of the Roaring Twenties, many others would become mobsters and become powerful off of alcohol.
1910-1918 = Depression Generation
Came of age either just before or during the Great Depression, and therefore would've been the most affected by it. Like the WWI Generation, they would've grown up with a naive worldview that was crushed by a certain event. They grew up as kids and teens in a prosperous time period following World War I, only to then experience adulthood during the Great Depression where they had to struggle to succeed.
1919-1927 = World War II Generation
Would've been coming of age or still in their early twenties during World War II, and therefore were the cohort most affected by it as they were in the perfect shape to fight in that war. Would've grown up as kids and teens during the Great Depression and would've experienced normalized hardship, so once the war was over and they came home to a prosperous nation, they didn't take it for granted, and would conform to society by creating the baby boom generation.
1928-1936 = Silent Generation
Would've all been under 18 by the time World War II ended; however like the previous generation would've grown up as kids during the Depression and World War II before coming of age in the latter 1940s and earlier 1950s , so would have also experienced normalized hardship and would've therefore also conformed to society as well as to the baby boom generation.
1937-1945 = Civil Rights Generation
Would've been coming of age during the pinnacle of the Civil Rights Movement, and would've therefore been the generation that contributed the most to that. Grew up in a prosperous period following World War II, and too young to remember the Great Depression, therefore would've been more critical of the 1950s status quo, which led many whites from this generation to support the cause for the Civil Rights Movement.
1946-1954 = Baby Boomers
Born during the post-World War II baby boom. Would've been coming of age during the mid 1960s to early 1970s during the Vietnam War, which was particularly unpopular among this generation, so many would join the Hippie counter culture and popularize bands and artists such as The Beatles and Jimi Hendrix. Are a lot more socially liberal and social cause oriented because of these experiences.
1955-1963 = Generation Jones
Often lumped in with the Baby Boomers due to still being born when birthrates were high, but this cohort would actually be a lot more conservative than their Post-War counterparts as they would've been coming of age in the mid 1970s to early 1980s, and therefore would've been a lot more affected by the Carter presidency than the Vietnam War. Would've also been big supporters of Ronald Reagan because of this.
1964-1972 = MTV Generation
Last generation to come of age during the Cold War, and would've witnessed the Iron Curtain fall during these formative years. Would be more conservative as they'd have more fond memories of Ronald Reagan. Would've been in high school for MTV, and would've popularized dance pop, R&B, glam metal, and hip hop music as a result of it. Also the first generation that would've used Personal Computers in high school.
1973-1981 = Generation Catalano
First generation to come of age after the Cold War. Are a lot more liberal than their Reagan Generation counterparts, as they would've been very fond of Bill Clinton due to Bush Sr's gulf war. Would've popularized grunge, Bad Boy - Death Row era hip hop, and pop-punk during their coming of age years. First generation to take video games seriously as they would've grown up as kids in the Atari and NES days and came of age in the SNES/Genesis and PS1/N64 days.
1982-1990 = Millennials
"Millennials" as they would've come of age at the dawn of the new millennium. Would be a very liberal cohort, as they would have distasteful memory of George W. Bush, particularly due to the Iraq War and Recession. Would've been huge supporters of Barack Obama because of this. Popularized 2000s Hip Hop (Eminem, 50 Cent, Nelly, etc.), emo, and alternative metal, and the first generation to use social media during these formative years.
1991-1999 = Centennials
"Centennials", as they would've grown up as kids at the dawn of the 21st Century, and then came of age during the formative years of the 21st Century. Are very diverse but more radical politically due to coming of age after Obama's election and in the time of the Tea Party Movement, Ron Paul wave, Crimea Annexation, ISIS, and Trump Election, so would therefore have more radical views on either the Far-Right or Marxist-Left. Would've popularized electropop, indie, and rap ballad pop, and first to use smartphones, tablets, and digital movie and TV streaming media during their coming of age years.
2000-2008 = iGen
The oldest of this cohort is just barely coming of age, so not much is known about this cohort, but will likely be heavily affected by politics due to growing up as kids and teens at the time of Trump's election.
2009-2017 = Generation Alpha
To be determined, as they're all only children at this point.
I'd also like to point out that this also all works out as a lineal generation theory, as one's parents would be from three generations on average before their own, being they would be 19-27 years older than the oldest in that generation and 27-35 years older than the youngest in that generation. Those older in the cohort are still likely to have parents from four cohorts back, and those younger in the cohort are still likely to have parents from two cohorts back, but to make a rough estimate, it would be a three cohort difference between parents and children.
So;
Centennials (1991-1999)
Parents = MTV Generation (1964-1972)
Grandparents = Civil Rights Generation (1937-1945)
Great-Grandparents = Depression Generation (1910-1918)
Great-Great-Grandparents = Edwardian Generation (1883-1891)
iGen (2000-2008)
Parents = Generation Catalano (1973-1981)
Grandparents = Baby Boomers (1946-1954)
Great-Grandparents = World War II Generation (1919-1927)
Great-Great-Grandparents = World War I Generation (1892-1900)
Generation Alpha (2009-2017)
Parents = Millennials (1982-1990)
Grandparents = Generation Jones (1955-1963)
Great-Grandparents = Silent Generation (1928-1936)
Great-Great-Grandparents = Prohibition Generation (1901-1909)
