First past the post elections. If we had ranked choice or runoff elections, more parties would appear.

Instead, in FPTP, every vote that is not for one of the two highest-polling candidates is objectively a wasted vote, since the game theory makes the possibility of a third party gaining enough votes to win in any single election nearly infinitesimal. So instead of many parties, all candidates self-sort into one of the two viable parties.

So what about primaries? The primary system decides the candidates, but even that is a game-theory-laden mess, because primary voters have to guess which will perform better in a FPTP general election and often vote against their ideal candidate in the hopes of winning the general.

In short, until we structurally reform elections to be ranked/STAR/runoff/etc to remove the punitive effect of voting for your actual ideal candidate, we’re stuck with a prisoner’s dilemma election every time.