Mike the Mad Biologist explains it was an attempt to cut into the business of the streetwalkers who used to ply their wares at the corner of Marlborough and Arlington in the 1970s:
People would hop off Storrow Drive and drive around the block, find, erm, a 'friend', and drive off. Not exactly the kind of tourism the city was hoping for, so the traffic pattern was changed. Hard to believe today, since the penthouse of Zero Marlborough (the corner of Arlington and Marlborough) recently went for $8.8 million.
He actually wrote that as an aside in a post about the new traffic lights that are going in at that intersection, which means the former red-light district is, finally, going to get some red lights.