Well, it has to do with the nature of politicians.

Politicians will say and do just about anything to be elected. Because of this, they are incredibly reactionary to what becomes popular, especially those politicians who are attempting to break into that large, unclaimed market - the undecided middle, that shifting, fickle sand.

A great example of this in Canada is the Liberal Party. They are the centrist party, and when they look at having an opinion on any kind of issues they basically just look at which way the wind is blowing, and follow it. their position changes continually based on what the popular ideal is, and that strategy has been very successful for them.

No politician can be elected without, in some way, engaging the great undecided middle. Hard left and right have already made up their minds, and their votes are already counted. So, a politician who gets elected (and has thus successfully engaged that middle) while espousing leftist ideals. Politicians looking to get elected see that success, and attempt to emulate it so that they too can engage the undecided.