If you compare the charts, the higher priced Nikons are sometimes of better optical quality. I know this is especially true for the nikkor 24-70 2.8L vs canon 24-70 2.8L (although still suffers from focusing issues just like canons) It is really back and forth so I guess that doesn't really answer it for you.

I haven't sat in front of charts comparing all Canon lenses versus Nikon lenses. But I know Canon has Nikon pretty much beat when it comes to the high end 70-200 range, and the two are nearly priced the same.

The answer is probably that there are hard core Nikon fans out there that would not go over to Canon to use a certain lens, so they will just pay what Nikon asks of them.