Not the person you originally asked, but the main reason is probably that referring to it as gnu/Linux is 1) already deeply associated with the Richard Stallman meme, to the point that referring to it in that way automatically comes across as either a joke or just a person being intentionally contrarian, and 2) just really weird sounding. In the minds of most people, there is no real reason to refer to it as GNU/Linux, because the actual operating system that does the things the operating system is expected to do - as in provide an API for syscalls, memory management, etc - is just “Linux.” That it’s routinely built alongside a set of core utilities designed and maintained by GNU is largely pointless. It’d be like referring to a hamburger as Buns/Hamburger or Buns+Hamburger. It’s just…weird.
Now, remember kids: if an unpopular Democratic presidential candidate doesn’t win an election, who do we blame? That’s right: literally everyone to the left of us. That’s how you make progress in a country, after all: Demonizing the left in the same way conservatives do. And why do we do that? That’s right, it’s because we’re secretly conservative, too. We just tolerate gay people more, but don’t want actual social or economic progress. Repeat after me: “unfavorable political outcomes are everyone else’s fault but mine!”