The Canadian immigration system, especially if you qualify for the skilled workers program (which most engineers do), is pretty decent overall, and much much better than the awful and outdated US system. I've looked at immigrating to the US and decided it's largely impossible. This largely explains Canada's immigration edge - it certainly isn't societal issues. I'd much sooner have immigrated to the US given the pay and tax differences.

I've actually heard good things about the AU system too - apparently Canada considers AU&NZ their biggest competition in attracting immigrants.