My own guess is that this is as much of an economic decision as anything else. I don't mean on President Obama's part, but rather on the part of the system. Oil at current prices, and with the current evidence of over supply and continued pumping and storing of additional oil, suggests that $50 oil may be closer to the norm for the forseeable future, than $100 oil.
Here's a pretty good article talking about the different moving parts in the economics of tar sands oil extraction:
http://www.onearth.org/earthwire/tar-sands-bubble
If we accept the numbers in this article, the tar sands oil is about $75/barrel for break even or marginal profitability before transportation to the market with demand for that oil. That transportation is about $9 / barrel via pipeline, or $25/barrel via train. With $100/barrel oil, or a strong belief that the price dip down to $50 is temporary, then Keystone XL is worth spending lots of money on (for the oil company / producer and pipeline company) to make happen. In fact, the pipeline is the difference between the project being profitable and not. At $50 though, the pipeline only changes the quantity of money being pissed into the big gaping wounds in the soil.
The moment you recalculate what oil is going to be selling for longer term and decide that's $50 (or lower), then the economics of tar sands development is gone, whether the pipeline is built or not. The pipeline company knows that, and they make money only if oil is moving through the pipeline. They (pipeline company) can and do lay off that risk with guaranteed contracts. If you're the oil producer, are you ready to double down on what's looking like a bad investment already, by signing a guaranteed contact that adds another $9/barrel for some number of barrels every year starting at some point in the future, whether you actually have barrels to ship or not?
Somewhere in here, the money stops working out. And at that point, everybody involved is ready to let it go. Oil producers, oil pipeline company, politicians (no more donations / interest from the first two), etc..
My own guess is that if oil went back to $100 in 4 years and we have a more oil friendly president in office, then the plans will get dusted off and we'll be doing this again. These things don't seem to die, so much as go take a nap.