There are several points to consider.
But the main one is this, many of the games bought on steam are full priced games that are only slightly cheaper then retail. Whereas the gaems people buy on xbla are usually far far cheaper.
Steam also has a very vibrant (probably more vibrant) indie and small budget market aswell. So its does well on both ends on the spectrum whereas xbl sells mainly inexpensive games and most of the expensive games are bought at retail.










