Fandom Is Different, sure, but I think there's two more specific factors at work. One, victims of trauma have more social capital to push for trigger warnings, and a pre-existing mechanism with which to express them (since warnings for spoilers and squicks are already well-established and supported by an even wider segment of the fandom). In contrast, real-world media aimed at a wider audience--well, there's no ratings system for books at all, and movie and TV ratings are vague, so there's no mechanism for it, and campaigning for one would be a zillion times harder.

There's also differences in terms of what, in the wider culture, we warn for/censor, depending on the medium. What's acceptable in a book is a no-no on film, movies can get away with more than TV (with a very vague rating off a single scale), a cable channel can show stuff that would never air on broadcast TV (but in the US, at least, TV has a richer rating system), etc. etc. How we as a culture relate to different media seems to dictate how tightly it's controlled, and fanfic seems to be in the same category as broadcast TV or radio--something that somebody could stumble into uninformed (or underinformed) and be surprised. Which is technically true of books and movies, of course, but we don't seem to find that fact as salient for those media--in fact, people tend to gravitate to the opposite assumption, that by making a choice to read/watch something you take a certain amount of responsibility for what you end up reading/seeing, so caveat lector. Does that say more about the nature of the medium or its place in the culture? I dunno.

Wow. Um, that was probably brainier than you wanted. Sorry.