Comment by graemep

Lots of things.

The US seems to restrict the ability of minor parties to stand for election far more than the UK does. It varies by state but from what I have read you need thousands of signatures just to stand in many states.

FTTP does also favour parties with a geographically concentrated base such as the Scottish and Welsh nationalists and the NI parties. Geographical variations in the US seem mostly to be about which of the two big parties people back.