You have to remember a few facts about Buells...
The SPortster engine has been the choice of HD Racers for 50 years because it has always put out more power per pound than any other HD motor, and that includes the VRod.
The Buell was designed around the Sportster motor and not designed like a lot of bikes are, around the frame and the motor being an afterthought. With the motor being a stress member and part of the frame, you can't just stick another motor in there either! It would require re-engineering the entire bike for a less capable motor.
Cost is also a factor. You can take a stock Sportster engine and get gobs of power out of it a lot cheaper than you can buy a stock VRod motor. The price of the Buell would shoot up by a hefty percentage and that is not even taking the R&R amortization into account.
Last, and to the HD enthusiast, certainly not least, is that the Sportster is pure HD while the VRod does not have the same heritage (sound, feel, history). Some people feel that it is a step forward, but a lot of us are not as enamoured with change for change's sake.