Rivet city and a massive robot.
Rivet City really didn't seem to build much new, they just took an old aircraft carrier and convert it into a living space. And Liberty Prime was, as I recall, a prewar project, just one that was never finished in time for the war, the Brotherhood of Steel just got the robot working.
Actually tech of dc Bos is better and it has most powerfgull squad in all bos lyons one. Weapons are better
Given that on the West Coast, they had T51B, whereas in Fallout 3, the only set available is not in the hands of the Brotherhood of Steel, I'd be inclined to argue that point.
All things said, though, the real reason the East Coast seems less advanced than the West is quite probably simply a matter of feel. Bethesda wanted to create more of a "scavenger" sort of feel, so they made the game the way it was, whether there's a logical in-world explanation or not comes second. Really, aside from matters of design, I see little justification for the East Coast looking like it does, considering the point the West Coast is on in Fallout 2.
Considering most of the US population lives on the East Coast, it's safe to assume that it got a lot more bombs than the west coast.
Which leads one to wonder, how are so many iconic landmarks still so intact?
And after 200 years, I doubt how many bombs were dropped would make much difference.