The short version is that the Constitution says the President is the one who gets the most votes. The Electoral college says there are only 548 votes, and they are mostly all awarded by each of the states to one victor (first-past-the-post).

The practical result is that if a party can only win 36% of the vote in a state, they get 0 electoral votes. Because of that, a two-party system has more or less been the norm.

It’s not ideal.