Since Boomers came before Generation X, why weren't Boomers also known as Generation W?

Because “Baby Boomers” was the first term that was coined

The name came from "baby boomers" - when WWII soldiers came back, they had a lot of babies around the same time. 3+ million babies were born in 1946. We see the same numbers now and our population is more than double what it was in the 1940s. In 1945 the pop was 139.9 million. It's 334.2 now. It was around 2.5-2.6 million per year before.

Because "X" wasn't a term that originated from the ordinality of the alphabet, but rather the idea of an "eXperimental" generation living in a new kind of society. There's also no "Y" generation and "Z" are usually so called out of fatalistic dramaticism. This is one if those cultural things that has so much nuance and potential depth to it that people write whole books about it, so don't expect to be satisfied with a simple answer.

When the phrase "Gen X" was coined, it was not coined as an alphabetical successor to another term. It didn't come after W. There was no W. Nor was there a R, S, T U, or V. Rather, someone called one generation by a name (Baby Boomer) and then someone else called another generation by a completely unrelated name. This happens a lot. There was no Windows 94 There was no Ford Model S There was no Covid 18 There was no iPhone 2 There was no Tesla Model R

Because they were named before we started using letters of the alphabet to name generations. Why would such a thing be retroactive?

The thing you need to understand about the generational naming is it's not part of some system which has been used for centuries to recognise and study generations. It's part of a system from a book written in 1991 designed to divide the population neatly into groups which would fit the two authors' theory (the "Strauss-Howe generational theory"). For the record, if you're interested, that theory basically says that all of humanity can be divided up into four revolving generations - one which experiences an economic and creative flourishing as a result of a post-crisis optimism, the next which becomes decadent and individualistic as a result of that flourishing, the next which comes to mistrust government because of individualism and where society basically turns on itself, and finally a generation which is so aggressive and has so little societal cohesion and belief in institutions that it causes the next crisis (usually a war) in its efforts to fix things. For the record, there's a lot going for the theory (and yes, obviously the theory says we are heading into the next crisis, which seems somewhat accurate right now) but there's also lots of holes in the theory - not least that pre-WW2 it only really works for the USA, even though it claims to define humans as a whole. But I'm getting sidetracked. Anyway, when Strauss and Howe wrote their book they talked about the various previous generations in order to explain and justify their theory. But at the time, Gen Xers were only between 10 and 26 years old and hadn't had enough time to define themselves in their impact on society. So Strauss and Howe just turned to mathematical naming convention, where X, Y and Z are used to name typical unknowns, and so named them Gen X. As for why we never gave Gen X a proper name...basically because it took a while for the book's ideas to spread enough for people to take notice. By the time it had, Gen Y was the generation everyone was talking about so that was the generation everyone wanted to name.

There was no systematic naming system for social generations back then. In fact, most previous generations weren't known by any name. Generation X only acquired its nickname after its members were already adults. Only the most recent social generations have population-wide nicknames. That trend was started by authors William Strauss and Neil Howe in their first book, published in the early 1990s. That book outlined their highly speculative and controversial theory of a four-stage cycle of generational archetypes. Strauss and Howe invented the term "Millennial Generation." The alphabetic sequencing of generational nicknames was something that emerged on social networking sites. It is entirely _ad hoc_, having been metaphorically pulled out of the derriers and soiled underwear of dozens of anonymous keyboard jockeys, and is not based on any known theory of society or social generations.