Difference is here we have more than two parties. The "centrist" Liberals are always in fear of losing voters to the left wing NDP, which is always anti-militarist. The size of the protests were indicative that a bunch of Liberal ridings would have been under threat if the Liberals had joined what was clearly going to be an unpopular war.

In the US, with only two parties to choose from, so-called "centrists" and "liberals" in the US almost all fell in line and nobody could do anything to punish them. "Who else are you going to vote for?"

Similar dynamic in the UK. Labour ("New Labour") was in power and supported the invasion for god knows what reason. Hugely unpopular move, but what were voters on the left going to do about it?