So one thing that just struck me big time just now is.....why didn't anyone pull a Mr. House and rebuild D.C.? I know unlike D.C., Vegas was spared the bombs, but with all those rubble, and concrete all over, someone could have easily organised a revitalization of D.C. or something, clean up the streets, use the rubble to build concrete walls. (Or use concrete to make walls, then fill the walls with the rubble to save concrete. I mean, D.C. has a lot of food in it lying around I'm sure, if people just pulled together, they could have made it a shining example of what Men can do when they work together, instead people are sitting in a broken aircraft carrier and a nuke impact crater.
Well setting aside for the moment the argument of "Bethesda screwed up" (which they sorta did). Here could be some possible reasons (but keep in mind there is usually a counterpoint behind them):
1. The DC region has heavily irradiated soil and water and thus makes it poor for farming (however we see a terminal in the game of a supposed agricultural commune which started early on so this sort of contradicts that). Assuming that Bethesda simply did an "oops" on the commune however, one can assume that the DC wasteland has largely remained something of a hunter-gather society with limited domestication. Rather than an agricultural one which is needed for full civilization to occur.
2. There is a sufficient lack of a good water supply in order to facilitate the growth of larger cities than what we've seen. Seems like many of the settlements tend to stick close to the Potomac area. Hence Jame's plan.
3. The super-mutants, raider gangs, and other assorted harzards are more dangerous than that of New Vegas. While the Super-Mutants don't seem to be that much of a problem in-game, its fairly apparent that the people of the DC wasteland live in constant fear of capture.
4. There is a definite lack of a "founder figure" in the DC wasteland. For New Vegas it was House, for the NCR it was Tandi, for the Legion it was Caesar. Not even Elder Lyons was able to cement his authority until the defeat of the Enclave and after he was shaken out of his 20 year stupor. Without coherent leadership for most of its time, a definable lack of organisation became the norm, with groups in-fighting and doing more harm to each other than good. If you look at Fallout 1, most of the towns are friendly and trading with one another for the most part. However in Fallout 3, one town wants to completely blow up another, one of the largest towns is a slaver post which is essentially at war with everyone else, and Rivet City appears to be maintaining a policy of isolationism and the Citadel certainly is. The rest of the minor settlements are simply too small to do anything but survive.
5. Other than these? The people of the DC wasteland are just moronic, which is why I would prefer if the Enclave just took over and began doing things the right way. Damn it the people of DC had 200 years to show some progress, they failed and it was time for a significant change. The Enclave would have undoubtedly given that.