IAD is UA's main airport in the area and it's not surprising regional jets pop up in/out of BWI.
From BWI to SAT, the obvious choice in terms of price, convenience, and plane would have been Southwest Airlines, plus you wouldn't have to change terminals like you probably would have to do at IAH w/ United.
With the change of a/c, you might have requested United to make a change to IAD as you origination and destination to/from Houston because those are almost always mainline. There's also 3 non-stops to SAT. Probably a bit farther for you to drive, but it is what it is...
Yes, If you insist on flying UA out of BWI instead of IAD, you're not going to get the better options.