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.