Why the heck would he do this in light of Trans Canada putting the project on indefinite hold as noted above? why not just simply announce that in light of Trans Canada's decision the decision here is on indefiite hold? why pick a fight over a materially moribund project?
Essentially if Trans Canada wants to bring the project back up in a more sympathetic administration, they would have to go through the environmental review process again because it would be considered a new project. This would mean new studies, public comments, and legal challenges. Meaning it would take years. So by Obama objecting to it, it either kills it for good or will require Trans Canada to have to go through the entire process again with new variables to deal with (NDP government in Alberta, Liberals in Ottawa, maybe an environmentally friendly Democrat defeats the Republican before the review is over and rejects it again).