Pretty good points.
Sometimes we want an explanation (simple or not) and there just isn't one (or at least it's unknowable, philosophers can debate the difference). Cosmos in the 70s? 77,691 at Giants Stadium for Cosmos vs. Strikers (I think it was Strikers). Why? At least partly because of the NASL stars: Chinaglia, Neeskens, Cruyff, Marinho, Carlos Alberto (I know not all Cosmos, nevertheless part of the League). Didn't Giants Stadium draw an almost full house for an LA/Red Bull game (Beckham vs. JPA)?
MLS may be perceived by those who control the relevant media as "second class", newspapers (and I use the term loosely around here) won't spend much time covering it, sports radio has plenty of other things to discuss. The word "contempt" comes to mind. This notwithstanding, there are writers who seem to like soccer, Mark Cannizzaro comes to mind, he's written a couple of pieces recently on Red Bull. My guess is it's an unpaid gig, though.
Again, part of it is star power. Populate the league with players that people around the world pay attention to you and New York will want to come to the party.
see NASL folds for regarding star power.
we have star power with Henry, Wayne Zlatan, Almiron Adams.