a) Almost all of the other incubators are just awful. Either they are predatory, ineptly managed or simply not well capitalised enough to offer a competitive product.
b) YC is able to rest on its laurels i.e. they can/do market how well AirBnb, Stripe etc have done. And so their funnel of talent is by far the best.
c) That said, YC is the worst I've ever seen it. And Garry Tan is mostly to blame although it's been trending downwards for a while. It's now as if Jake Paul ran an incubator: very hype driven, cynical, focused on easily exploitable young 20s males.