Here is the basic thought. Harpers attack adverts work. They suppress the vote. Harper also makes it harder for poor people to register to vote. He is not going to stop. His new prisons will be populated more with Natives and Minoritys, (again people who vote more ndp than conservative). So he works to supress the vote of those who vote against him.
In Australia, it is compulsory to register and it is compulsory to vote. (They have a none of the above choice when they vote) and they also get a nasty fine if they don't vote. (remember that we already get 3 hours off work to vote) Thats a nice carrot, but clearly a stick is also needed.
Lets face it, Harper is never going to stop making nastier and nastier attack adverts to prevent people voting. It will just polarize canada into voting conservatives and non voting ndp ers. Compulsory voting is an easy way to increase the voting numbers and render Harpers attack advert money meaningless.