It's down to a contractual agreement with Sonisphere. Sonisphere had them down as UK headline exclusives, and Glastonbury effectively had them booked for the Other Stage on the Saturday night, with Prince headlining, but with Prince, not giving a definitive answer and the bookers not wanting to miss out on any headliner at all, they've pushed up Metallica to Pyramid headliner, but can't announce as it would break Sonisphere's 'exclusivity' clause.
Will do in due course.
Mind, if they were originally booked to Headline the other, i see no reason why they couldn't be rebumped there, but seems very unlikely, that Oasis would go to Glastonbury as opposed to like 10 nights at Heaton Park.