Because they're very smart.
I have to say that I fundamentally do have huge personal problems with this, but why I find that I could support it is that I don't think this changes anything. In substance, you still have the ability of the Prime Minister of the day to go to the Governor General and ask for an election, so I think this is more window dressing than reform.
Fundamental for me, if we did go to a constitutional amendment to actually effect this so that it would have real teeth, would be the fact that members of Parliament are appointed and are really responsible to the House of Commons. So for a government, especially when we seem to be going for minority governments, to either reach an impasse or truly lose the confidence of the House, I don't see this accommodating that. To me, that's where the real accountability lies, in being responsible to the electorate. I don't see that being accommodated if this in fact substantively did fix it at a four-year interval.