I think maybe because people think of "festival headliners" as big rock bands, and there arent really that many current, new big rock bands worthy of headlining, there are a lot of upcoming bands however imo. The likes of Royal Blood may perhaps become a headline act one day who knows, i dont think rock is as mainstream as it used to be i think Rap and sort of nicki-minaj-type-**** is the mainstream at the moment, and i think festival planners are reluctant upset the applecart and put someone like Nicki Minaj as a main headliner.. gladly so, id much rather see a has been who has created good music than a forge of artists such as her and the like