Animators: "Yaaaay! We finally finished our passion project! A serious, mature, adult oriented animated feature with no porn! We won't make much money from it, but who cares, right!? Now... let's focus on our next project: To whom do we sell our studio?" :)
There are animated movies that go straight to DVD or TV and they seem mature enough, even though some use already pre-established characters like Batman. Spawn was one for a while but I think Aeon Flux struck me as the best example of a mature, animated show that was not geared for kids at all.
Oh yeah, Aeon Flux was awesome, MTV had 'The Maxx' around the same time, that was pretty good too
What about web toons? David "Salad Fingers' Firth is pretty awesome with a range of cartoon which Salad Fingers is only at the more extreme range of (although Spoilsberry Toast Boy and a couple of others probably go slightly further, admittedly)...
The Maxx was awesome, and The Head was often great. Aeon Flux was complete crap though, the animation was bad, the writing was bad, and I'm still not sure how anybody could like something that was so bad on so many different levels.
Perhaps it's because I'm familiar with what good animation looks like. The old WB stuff, the earlier Hannah Barbara stuff, pretty much anything that John K is involved with. Hell, even Disney had Talespin, Gummy Bears, Rescue Rangers and Ducktales which all had quality writing and quality animation.
As for the question, the problem is cost and the perceived notion that there isn't a sizable market for more adult animation that isn't just a cover for pornography.