What we know is that MK usually wants to control everything, and in a huge festival like this one where every name is big, MK wouldn't have a say in when, how and with whom to play. I think that's maybe what made him refuse.
For instance the Nîmes summer festival, where Mark appeared a few times, usually programs several artists a night but when MK plays there it's only him for the night because he is a big name and can impose his conditions, namely I want the stage for my band and crew only.