i think funding is huge for a lot of people, and funding can compensate for a lack of well-known faculty / well-known alumni when it comes to reputation. i think uva and jhu are the best examples of this -- not to say that they aren't fantastic programs on their own, or that their faculty/students aren't spectacular, but more so that people are willing to apply to these even if they haven't really heard much about their faculty/alumni, because their great funding allows them to be super selective
some others, like syracuse, are fully a faculty thing. people love saunders enough to overlook the funding i guess? but in general, for programs in boston or new york, unless they're exceptionally well funded, the stipend is usually only enough to barely get you buy, and sometimes not even that (depending on your offer)
others are well-reputed because they're old. so if you take a program like iowa that's been around for a hundred years (and that too, was around before any other programs were around), it makes sense that tons of good writers emerge from that program because 1) for a while it was the only one, and afterwards its reputation drew well-known writers back as faculty, and 2) the fact that it's been around for so long and that its cohorts are relatively large means it's churned out a LOT of students, so naturally we would hear some percentage of them even if the majority aren't major literary superstars