https://www.eia.gov/outlooks/steo/images/Fig19.png is a decent illustration of one of the issues here: the US has been reducing its demand for oil imports over the past decade. In addition, the kind of oil that comes out of the tar sands in Alberta is pretty much the lowest, worst grade of oil possible (not to mention some of the most expensive to produce!), so if you've got more oil than you know what to do with, it's the first one you'd want to stop buying.
Besides the environmental issue of "we don't want to spend any extra money subsidizing oil production," there's also a strong amount of concern about the impact along the construction pipeline--particularly among the indigenous lands it would travel against the wishes of the indigenous people who live there.
Besides the environmental issue of "we don't want to spend any extra money subsidizing oil production," there's also a strong amount of concern about the impact along the construction pipeline--particularly among the indigenous lands it would travel against the wishes of the indigenous people who live there.