Looking into how Marvel and DC have been trying to set up 'Shared Universes' longer than most people realise, I found out that an earlier attempt at a Batman vs. Superman film was mooted because the actor being considered (Christian Bale) had already been approached to play the role of Superman by another director (Darren Aronofsky), in another version of Batman.
The original production (to be directed by Wolfgang Petersen) was greenlit by the studio, but couldn't attract Bale, who was attached to another (ultimately unrealised) Aronofsky's Year One adaptation... but how can this make sense?
Warner Bros own the rights to the Batman character, so there is no way to make a film without entering into negotiations with them. If Petersen's film was greenlit, then he must have obtained permission from DC. If Bale wouldn't walk away from Aronofsky's version, it must also have had a shot of being made, or even green-lit: if DC didn't want the movie made, all they would need to do is withhold the rights.
It stands to reason that DC was in negotiation with both directors simultaneously, but if they greenlit Petersen's film it would no longer be in their interests to support, or even entertain, Aronofsky's. Development should have been cancelled immediately, freeing Bale up to move onto Petersen's, should he wish to.
Why would DC entertain two competing productions?