This in part because both major political parties use red-white-and-blue as their color scheme (which actually tells you a fair bit about American politics and is suggestive of the differences -- or lack thereof -- between the two major parties). Thus neither party has a "natural" color that distinguishes it from the other.

However, entirely by chance AFAIK, the networks were using red as the Republican color and blue as the Democratic color on election night in 2000. The long, drawn-out drama of that night has, for the moment at least, turned red into the Republican color and blue into the Democratic color.