I think it's because the studio (in the case MGM) paid for the rights to film it. They have 2 choices:
1) Film the next Bond film soon.
2) Hold onto it until their rights to the franchise expires.
And if I'm correct, the producer only has the rights for a limited time as well (unless he actually owns it 100% ). Eventually it goes back to the actual owners of the Bond franchise and the producer will have to negotiate a new deal with them all over again - costing him another fortune. It's a lousy predictament to be in when you want to film something but can't - and the clock's ticking away at the same time.