When the United States joined the Allies, why did the Allies concentrate first on defeating Hitler?

Military strategy.  The main allies we had in the war who had not yet been conquered, that is, Britain and the Soviet Union, were most directly threatened by Hitler, not Japan.  So in a concession to them, and to hold the alliance together, we agreed at Casablanca (and earlier at the Arcadia Conference) to fight Hitler first and Japan second.

It also makes practical sense, in that if the Soviet Union or Britain were conquered while we concentrated on Japan, then the alliance would be much less effective sans one member.  Japan could be held at bay indefinitely, as they had no way to invade the United States proper.  By defeating Hitler first, we would then gain valuable allies (we thought at the time anyway) in defeating Japan once that was done.