Here's the deal: DC real estate is some of the most expensive (and non-existent) in the country. DC United has been lobbying and looking for a place nearby forever. I've been to RFK.....it should have been demolished years ago. Even their guys joke around about the racoons, rats and other vermin that live there--and it's no joke, I've seen them there. When we go there, we are lead by flashlight up to the upper balcony as there are VERY few functional lights above the first floor. There are no concessions nor bathrooms above the first floor either, and the fatigue of the stadium is obvious. DCU could never raise enough funds to simply tear it down and build a new stadium, and the local government has not been the most helpful to keeping them in DC.
All of this is actually quite sad seeing as DCU (while I hate them) are to be respected as they are historically the most successful (trophy-wise) team in MLS. Hell, they even won the CONCACAF Champions League (although at the time it was a different name).
Baltimore is relatively close to DC, but that would displace a fair amount of their fans--not to mention disenfranchise them. Imagine if they moved the Fire to Gary, IN or Milwaukee.......it would piss off a lot of the fanbase and make it difficult to trust/want to be around the organisation.
Put it another way for all you Man United fans.......what if they said "hey, Manchester isn't working out as a viable place for our team.....let's move them to......Liverpool. I mean, it's pretty close, right?"