Because the middle class refuses to change it's voting habits, and voting is the lazy person's habit of not getting involved in their participatory democracy.

It isn't just in the US though. It is just that in parliamentary system, you have more choices.

Here, you also have roughly the same number of choices as the parliamentary democracies, but no one is allowed to talk about them, and they aren't allowed to participate in the elections in a meaningful way.

People actually vote for them, when they are allowed in, as both parties found out in 1992 and 2000.

And maybe this year, it's hard to tell because the censorship of the third parties is so blatant this year.