Because the Canon is more expensive!!
I read a couple of reviews between the two that say that the Canon is a bit sharper. That may be imaginary, and even if so, it's probably negligible. Actually, since I plan on using it with the 1.4TC, the zoom function of the Canon should come in handy. That's really the only reason.
Plus I found my 8-15 used for about the price of the 15 new.