For the presidency, the US has a double first-past-the-post system for a single seat. Electing a minor party requires winning a majority of a pluralities: a plurality in enough states to get an electoral college majority. That's a very tough task, somewhat harder than trying to elect a Green party candidate nationwide if all Canadians voted for a single "Prime Minister Seat."
Parliamentary systems like Canada also do more to encourage minority parties at the per-seat level, for a few reasons:
- In a minority government, like Canada has seen for much of the last decade, minority parties like the NDP and BQ really do have legitimate power to shape the national agenda.
- In a majority government, nobody expects the opposition parties -- any of them -- to have much if any influence on the agenda, so to first order it doesn't matter what party you vote for provided it's not for the nationwide winner. Strategic voting does affect this, but it also cuts both ways if a minority party puts out a strong, local showing.
- Party discipline is also much stronger in Canada than the United States, so the parties occupy correspondingly smaller ideological grounds. In the States, a Republican in New York City is not necessarily the same as a Republican in Alabama, and a southern Democrat will still tend to be more conservative than a Northern counterpart -- and this really does influence legislation, to both good and ill. The upshot is that third parties are less likely to get a consistent regional base in the States, since the local duopoloy will incorporate the regional idiosyncrasy.