Because it removes the oversupply:
“Existing markets for Canadian heavy crude, principally PADD II [U.S. Midwest], are currently oversupplied, resulting in price discounting for Canadian heavy crude oil. Access to the USGC [U.S. Gulf Coast] via the Keystone XL Pipeline is expected to strengthen Canadian crude oil pricing in [the Midwest] by removing this oversupply. This is expected to increase the price of heavy crude to the equivalent cost of imported crude. The resultant increase in the price of heavy crude is estimated to provide an increase in annual revenue to the Canadian producing industry in 2013 of US $2 billion to US $3.9 billion.”
That's from TransCanada's permit application...the above is TransCanada telling you precisely how energy costs for the US will rise.