Well, since you've got sites like bridgebase.com, maybe Microsoft decided to just stop spending money on their own version of Bridge? Much like how they decided to stop supporting Encarta with Wikipedia around?
Then why do they continue to run the other hundred games that they have? I don't think that is the answer. MSN bridge was the best site out their. Even after they stopped supporting it, people still flocked in to play until they turned it off completely several months later. The other sites are VERY poor quality and lousy graphics etc.