Obviously, take my words with a grain of salt since I've never lived in the US.
But coming from someone who's been there several times and got family/friends in the East and South, from the outside looking in, I think there's more coastal insecurity than NY bias. NY created hip hop. I think the general attitude in NY is "outdo us or be on par with us, and we'll give you props." So the acts from the other coasts that were obviously delivering dope shyt (e.g. Snoop, Ras, Face, Outkast), they got love. The dudes who weren't up to par got dismissed. For a long time, the East Coast was dominant as fukk. To be honest, NY tastemakers seem like they were giddy that hip hop was spreading as opposed to staying in NY. So I don't get this idea of "you, stay there" attitude that other coasts accuse NY of. I'm sure it applies to some NY guys but I'd argue they were in the minority. And the other coasts simply weren't up to par. Real shyt.
But as the coasts started getting better at making hip hop, particularly The South, NY embraced their shyt. Timbaland imo is the guy who really put the entire South on because his bounce is the perfect bridge between East and South. And producers out here in Toronto (East Coast-ish) loved Timbaland. NY must have loved Timbaland because he worked with their major rappers and helped define their careers.