Because they figure they'll make more money with the Nolan film series, and any dodgy television show (because, let's face it, this is the CW) will only harm the brand (and potential box office).According to Warner Brothers they will never authorize a live action Batman television show. I remember reading that they were pissed when he was shown for five seconds on Birds of Prey and when Alfred called him on the phone in the final scene of the series finale.
Why won't they allow a live action Batman series? It would be a money maker (presumably) for them, and I can't see a studio turning down a chance to make money.
Is this the logic for why Superman Returns tanked? The exsistence of Smallville on TV? I always thought it failed simply because it was a bad film.