I would say craft and the desire for artistic achievement first and foremost, but commercial developers (like all creative professionals) also need to consider how they're going to make money when they create a game. Only people who make games in their free time and distribute them for free or developers who have made so much money on other things they can afford to finance a pet project can realistically do it for the sake of art alone, but I believe that the games produced in this way tend to make the experiments that end up eventually changing what we see in the big budget titles that sell a lot of copies (which is not to say that such titles never innovate, but it's more usual for them to simply refine, or alternatively not be very good).