Why? Well, WTOP reports in vague terms that it has something to do with increased efficiency (Works for me).
The Circulator, which the D.C. Department of Transportation runs, already includes several routes around the city, but adding this one might be tought. According to WTOP:
There are some legal issues to tie up first—crossing the Key Bridge would add buses to what is an already congested Potomac Crossing. The D.C. Council would likely have to weigh in on the proposal before the expansion to Rosslyn could happen.
DDOT replaced the Blue Bus, officially the D.C. Metro Connection, thta runs between Georgetown and Rosslyn with the Circulator as part of a “pilot program” in 2007. Guess it didn’t stick.