In my mind I handwave it as the east coast got so fucked by the nukes that there was next to noone left there, and only over time people filtered back into the eastern seaboard from surrounding areas, which is why the west coast is so much more developed. I know that's not a good explanation, but it's better than the lack of one we get from Bethesda.
EDIT: Plus the more hostile environment of the east coast. Because apparently super mutants and zombies feral ghouls are everywhere, which might dissuade people from resettling the area. Not to call the Yukon a hostile environment, but it had more people during the gold rush than it does now, and it's taken over a hundred years for the population to get up that high again after the population dropped to the low thousands following the gold rush. So it makes sense to me if I squint my eyes and look at it a little funny. But that's just my headcanon. Thinking on that makes me wonder about how Whitehorse/Yellowknife are doing in 2287. There's a lot of isolated sizable communities in BC and Alaska as well that wouldn't have gotten hit by nukes. Plus Hawaii I guess. And I'm sure Newfoundland/Labrador weren't prime targets. And that's just considering the US and Canada. The rest of the world wouldn't have been hit as hard, and I'm sure there's a lot of big communities left around. But that kind of stuff is never touched on in these games or the Fallout Bible IIRC, which is a shame. It'd be a fun thing if 2287 America was raided or threatened by groups from Africa/South America/Northern America/the Pacific Islands like that one book series with the Haida raiders and the British raiders in Post apocalyptic USA.
And here I am rambling, I'm putting way too much thought into this.