Because it's the least locked down by rich people, basically.
It would have been much spicier to title this "the most legitimate" esport but, honestly any big esport game you can think of is probably perfectly legit. There's a long and storied history of new sports coming into existence and being corrupt, predatory, and or toxic as fuck for the first few decades. That's just normal. Taking advantage of the players isn't surprising in the context of a new sport. It sucks shit and I wish it didn't happen, but it can't be used as an argument that a sport is not legitimate, if anything it's the opposite.
Speedrunning isn't really beholden to any single group of people. There's no single publisher that can try to stamp out people playing a game in a way they don't like, or make sweeping fundamental changes to the game however they see fit. There's some organization and structure but should any major component of that make a Bad Choice, it can't really kill off the whole sport at once. It's a much closer equivalent to a ball and a bat than say, DOTA, to me.
FGC is kinda close to be fair, and makes better use of the competitive aspect. But there's been some occasions where publishers have stepped in and shitty'd things up for one reason or another.
Ironically the lack of... centralization, I suppose, makes speedrunning a lot different as a spectator sport. You can't really have a single speedrunning championship, since by definition it comprises of drastically different disciplines. Going and watching your favorite runner is like going and watching a very local sports team, and events feel more like exhibitions than fierce competition. But I think one can look at that and say that makes it a very pure sport, since the actual play matters more than the organized aspect of it.
