Because people are failing to vote strategically. In Canada the 3 party system even more heavily favors the right (Conservatives), as our center-left parties (NDP, Liberal) are very similar and split the vote. In strategic elections this narrows considerably.
We also have a parliamentary system, where these parties can make coalitions and essentially pool votes, which softens (but doesn't eliminate) the need for strategic voting.
Canada has 5 parties. Bloc, lib, con, ndp and green not to mention another protest right party. A few years ago the cons were split into two parties.
Rarely outside of BC does lib-ndp vote split work in the con favour and often works the other way for the lib in Quebec with the bloc.
The ndp and lib seem like they are progressive and the idea that one should vote for the other to block the cons misses the differences. And your vote is worth a few dollars so vote for what you want... because funding matters.
We also have a parliamentary system, where these parties can make coalitions and essentially pool votes, which softens (but doesn't eliminate) the need for strategic voting.