Or, spare money. Which is totally understandable given the necessary time (=money) to create stunning state of the art graphics. Innovation in the actual gameplay is actually much cheaper, and might easily be better for the player in the end.
Although I admit I prefer some middle ground, not explicitly retro, Braid is a good example.
Edit: 'spare' a bad word. It's so expensive that makes the difference between feasible and impossible development projects for the small studio.