Parliamentary systems, even with FPTP, are inclined to have more than two parties. The centre-left is a much bigger portion of the electorate than the right in Canada, so I think it makes sense that we'd have more than one.
I would say rather that all inclusive political systems (ie. not specifically parliaments) are inclined to more than two choices, since it's always impossible to bind multiple binary (or more) axes into a single binary axis.
Systems that divide one winner-take-all battle into many smaller winner-take-all battles (including FPTP and single-winner runoffs) are what apply pressure towards shoving all that diversity into a single binary axis. Systems like the US president are particularly binary because the outcome is singular on both levels: individual electoral college seats and many states are winner take all, and the whole contest results in the election of a single person even with a plurality outcome.
The pressure that allows Canada to escape this tendency towards binary choice is almost certainly a rather extreme regionalism. All successful third parties in modern Canada (since the Liberal/PC axis was established) have arisen out of regional pressures, afaik. Socred from western religious right, NDP/CCF from western socialism during the depression/dustbowl, Reform from prairie soft nationalism, Bloc from Quebec hard nationalism, and the Green party from BC environmentalist concerns. The US hasn't had that kind of regional pressure since the civil war, probably largely because it was recognized as something that nearly ripped the country apart so it's heavily discouraged from becoming a formal thing (even though obviously there is an implicit south/north and coastal/middle divide still).
But on the second half of this, I agree absolutely. Much of the difficulty here is our tendency to frame our own politic in US terms, so we talk about two centres: the one that floats to the left and right of the Liberals, more or less, that identifies a Canadian centre, and the American one that floats to the left and right of the Conservatives. So we see two parties to the left of the American centre, but really what we have is one party on the left, one on the right, and one that lives in the mushy middle ground.
