Changing demographics (which will only continue to change), as you said being the main reason. The people who tend to vote Republican have decreased to the point where they have become irrelevant, and none of the other parties are large enough to stand much of a chance. I think in the future we might see more battles between different "flavors" of Democrats, since the people who vote democratic tend to come from many different groups who may not all want the same things.

I guess we'll see.