Wallu wrote: Well we have hads Springsteen, Prince and The Rolling Stones recently so that gives a little hint that U2 isn´t imposible.
Yeah, but that's viewed entirely from a RF-only standpoint based on a notion of "look, we also had THESE legends, so why not U2?". If you look the other way and see it from the band's POV, they're really not very well known for playing festivals.
In fact, the Glastonbury gig in 2011 is still their only festival gig since 1985, whereas the three examples you mention all have played festivals more regularly than a single concert in 30 (!) years. Admittedly, Rolling Stones' festival appearances in the same timeframe can be counted on your fingers, but they're still way more used to the festival circuit than U2.