The reason why the Marvel Universe revolves around the East Coast, and more specifically New York City, is simple: the company was founded and ran by East Coast natives, many from NYC or near it. The Trinity of Stan Lee, Jack Kirby and Steve Ditko itself is an example, with Stan and Jack being native New Yorkers and Steve permanently migrating there from Pennsylvania. The effect was "write what you know", hence why so many superheroes are from NYC, or set up their base there, and of course it made it easier to organically have them cross over with each other. I don't think there's a comic book company that embodies the East Coast culture more than Marvel. Really, it's hard to name an entertainment company in general that does, even DC which was also NYC-based until recently (they relocated to Burbank, CA in 2015) doesn't feel as defined by it.
This is actually part of the lore itself. California, and the West Coast as a whole, is very light on meta activity, compared to the East Coast being a hotbed of it. Runaways for example hinged on this idea, and this has been acknowledged many times.
But really, if Stan, Jack, Steve and the others grew up in California, then Marvel would've revolved around it instead. It's that simple. So what if Marvel instead revolved around the West, and Los Angeles was the core of their stories? What changes?
Thought this would be interesting.