But Bale requires the least amount of setup...he's already established as Batman, and comes with a tailor-made batsuit and potentially an established supporting cast (if needed). The setup doesn't need to go past a throwaway line or two to fill in those who've been paying attention to the Nolan Batfilms.
Putting more thought into it...even if they want to get Bale, it's a win/win for WB to say that they're recasting. If they don't get him, it won't be seen as a disappointment and a pre-production failure. If they do manage to get him, it can be played up as a major coup, and everybody looks good.