1. Cost of different programming teams for different consoles, and
2. Seller fees imposed by console makers.
For example, if I made a game for XBox, I am required to pay for development of course. But then I ALSO must pay Microsoft a HUGE up front fee to use their name and logo on my box, as well as pay them a large portion of profits. PLUS I must pay for the physical pressing on the disks, the boxes, the artwork, the shipping, the marketing.
BUT, if Microsoft were to drop their fees altogether AND allow developers to sell their games in the XBox store for download to the console, I think we would see a BOOM in XBox games... making the XBox a much more attractive console.