I suspect it's mostly because there's no All Media Types or & Related Fandoms tag. In previous fandoms I've been in like that, while there was a certain amount of rampant crosstagging, people who were writing in a sort of blended fanon universe just tagged AMT. Which was cool because people who didn't care were probably filtering on that one, or a pairing tag, anyway. But the wranglers have stopped making AMT tags for newer fandoms, it seems like. Which I'm sure is helpful to both the wranglers and the servers, but doesn't really match up with how people actually *do* fandom a lot of the time.

(It was an issue in GO fandom too - no AMT tag, so people tagged book + show for everything that wasn't 100% one or the other, so it became very hard to find pure bookfic.)