Just ease of administration. IME 9/10 will eat the granules with no problem at all, but fewer than half will eat the pills without a bunch of gymnastics to hide or disguise them. I don't have time for that nonsense.
I was wondering this myself a few months back. I stuck with the granules though because they were enteric coated which if what I have read on the subject has been correct, is necessary to get the drug to the appropriate point in the horse's digestive system for it to be effective.