First past the post doesn't explain Canada's four main parties. There is something preventing additional parties in America
The US system wasn’t engineered with the possibility of coalitions and compromise governments in mind (at least not in the Executive).
If the US president was elected by the Congress would that end bipartidism? I don’t know.
When you have more than two you could end up in a minority government where a smaller parties decide issues by supporting a big party.
Provinces have had other parties become the government.