By taking over the Progressive Conservatives, Harper was able to end vote-splitting on the right. This gave the Conservatives a major advantage over the Liberals and NDP.
In theory, the Liberals and NDP could have compensated for this weakness through strategic voting, coordinated through websites such as www.projectdemocracy.ca: anti-Harper voters would have voted for the NDP in NDP-Conservative ridings, and for Liberals in Liberal-Conservative ridings.
In practice, though, strategic voting is just too complicated for people who don't follow politics closely. Frankly, politics can be extremely boring. So-called "low-information voters" take their cues from the media, which focuses on the leaders; they often don't know which riding they're in, or who their local candidates are.
It'd be great if everyone paid close attention to politics, but that's unrealistic.
Consider Vancouver South (where I was volunteering yesterday). In 2008, Ujjal Dosanjh (the former BC premier) won the riding over the Conservative candidate by 20 votes, with the NDP a distant third. If there's any riding where strategic voting against the Conservative candidate would be the obvious choice, Vancouver South would be it. Judging by lawn signs, the NDP certainly wasn't campaigning hard in the riding. Yet the NDP vote in Vancouver South increased from 2008 to 2011.
The vote-splitting problem for the NDP and the Liberals is likely to be aggravated further by
the Green Party.
In addition, the strategic voting sites aren't that reliable. In particular, they didn't do a great job of identifying close races. A post-mortem from James McKinney.