That doesn't have to be the case - the differences between e.g. Southern Blue Dog Democrats and East Coast liberals is big enough that they could very easily have split into different parties, each being strong in a limited number of the "50 nations", even with a first past the post election system. Except that Congress has some strong incentives for aligning into just two parties, so even someone like Bernie Sanders, who purports to be a socialist rather than a Democrat, acts as if he's a full member of the Democratic party - and runs in their primary. And only in the presidential or gubernatorial elections do you see third party candidates standing any chance at all - and still not much of one, since they can't get any support from people in Congress.

Don't get me started on the drinking age - there is precisely one state in the US that I'm aware of which is even remotely sane about the drinking age, and that's Wisconsin.