The festival gets the 3 biggest headliners they can generally - and avoids repeats where possible.
It depends on how you define megastar i guess but clearly SZA didn't pull the crowd of a megastar this year, nor does Stormzy sell shows like a 'mega star' would (in my opinion at least).
Just because I discuss whether an act is big enough or not doesn't mean it happens. Point of a forum is debate / discussion
Wait what...... Are you not the user on here who spends your time policing anyone daring to suggest a headliner that is not a superstar or mega band! Its like your whole deal???????