CP didn't have a choice. They have been bought out by Air Canada, a STAR Alliance member and one of the conditions in the sale is that CP must leave OneWorld and join STAR.

If CP had the lifeline it asked for from current OneWorld members (excluding American Airlines), and there was some loyalty and concern, then maybe OneWorld still have some representation in Canada.

But alas, it seems that the STAR allegience was far stronger than OneWorld.