Other than the technical challenges of programming for multiple cores, commercial games have to run well on low end systems w/o multiple cores to make money.
Now that multi-core processors have been out for a while and the major game consoles have multiple cores it's only a matter of time before dual core shows up on the minimum system requirements list for PC games.
Here's a link to an interview with Orion Granatir from Intel where he's talking about getting game developers to take advantage of multi-threading.
