and DC plans on waiting until they've officially finished with the MoS franchise in its entirety, the only way they'll be able to use the Henry Cavill Superman in a JLA film or associate him with such a film will be if Nolan stops his association with the MoS franchise as a producer.
I wasn't sure if it's confirmed Nolan would be around for the sequels, honestly.
Simple answer? Money and the ability to market a new series of Batman solo films set in the Nolanverse as being set in the Nolanverse. Being able to piggyback a new series of Batman solo films off of the success of Nolan's trilogy by touting that the new films are set in that same universe would be very helpful in generating marketing for said new set of films, which would lead to more money.
I really think that's overstating the relevance of the universe as
Christian Bale as Batman, Heath Ledger as Joker, Christopher Nolan directed, sure, all those things matter.
But a brand new Batman who is however also still in continuity with the older Batman? Less so. In this case the bankable property is the new Batman
- like the new Spider-Man. The 'Nolanverse' is defined as Nolan's specific take, and his beginning-middle-end, on the Batman mythos. I don't think a new Batman movie would do better just because it says it's set in the same universe.