I'd guess it's at least partially due to the continuing perception (Adult Swim notwithstanding) amongst broadcast execs that animation's for children. Whilst TAS was indeed more or less aimed there, the franchise has become such big business that they're unwilling to take even that step. But then, TV animation always has a tough time, outside Asia. *sigh* Not that there aren't exceptions, eg Adventure Time and MLP:FiM (~yay!~), but even Warner's been reluctant to venture far back into animation, despite Batman's success, let alone TTA and Animaniacs.