One reason is that Ultra Violet codes are available for more movies and there are more people using UV than iTunes. Just supply and demand. For example, there is not a iTunes code for San Andreas, Pixels or Vacation.
Yeah Disney kind of got it right.....wish the entire industry would work like that.....buy anywhere/watch anywhere.
I've stuck to mostly iTunes purchases in the past just because I never pay full price. I always have iTunes money that was purchased at either 20-25% off.
-Kevin