Probably it's because the Capital Wasteland is truly a wasteland, with constant cloud cover, pervasive radiation, utter chaos and things waiting aroun every corner to take your life.
The Mojave feels more like a place slowly decaying with time, but not truly a post-apocalyptic wasteland ... there are large settlemtns, casinos, plenty of places to rest, blue sunny skies .... not really the same sense of utter despair that Fallout 3 captured so well.
In Mojave, they're irrigating crops and gambling, worrying about NCR politics
In DC, they're trying to stop vampires, Raiders and Super Mutants from taking away what little they've left.
I'd be pissed off if I lived in the Capital Wasteland, too.
I agree with this. Plus, the western portion of the US has been patching itself together for the last 203 years, so it's natural that things are looking better as of 2281.
The Capital Wasteland, however, site on the Tidal Basin, which is very irradiated, so process patching things up are slow. Not to mention the hundreds of ruined buildings from Pre-War DC.
So, it wouldn't surprise me if the LW wasn't nice. (Plus, consider that he/she grew up in a vault where pretty much everyone treated him/her as an outsider, especially the Overseer and the Tunnel Snakes.)