I think it was naïve promise Trudeau made when the party was in third place in the polls thinking he would never have to implement it.  At the point it was made, few thought the Liberals would have much chance at getting a majority.  I don't think Trudeau deliberately planned to break it anymore than Chretien planned to break his promise to scrap the GST, I think once he saw it was a lot more difficult than he thought, that forced him to change his mind much as Canada's finances in the 90s forced Chretien to keep the GST.