In part because voters don't vote for those who maximally represent their interests. It's a rare opportunity to vote for a candidate I support; normally I vote for the least-bad, simply because a candidate I support isn't there.

Voters may prioritize one (or few) cause(s) over others, leaving them disenchanted when a low-priority issue gains importance.

If the American system hadn't been successful for centuries, I'd have guessed that it wouldn't work.

The primary system, where candidates for the general election are selected, emphasizes the core/extremal parts of each party. The general election emphasizes centrist views. The usual result are candidates who will tell people what they want to hear. This makes it more difficult to predict what a candidate will do in office. Furthermore, a candidate who aspires to greater office may not do an optimal job in his/her present position in order to tune appearances for the coming election. Term limits would go a long way toward addressing this latter problem.

We get the government we vote for, and we don't always (normally?) vote for those who might do the best job.