I'm a huge fan of animation, same as you OP and this question has bugged me for a very long while as well. Heck I even made a thread similar to this a while back. Since then I've though about it and I believe I have part of an answer.
Really it all comes down to money. Animation is crazy expensive, as a single scene that lasts only minutes can take hours or even days to draw. Thus Animation studios need to make as much money out of their production as possible. To do that, they aim for all audiences. The only people who could possibly make mature cartoons are those who are not interested in making an explicit profit, and are instead interested in bringing people to their service. It's why I've always said that HBO, Netflix, Hulu, etc will be the first to do it. Heck, HBO has already made the most mature TV animation in America with Spawn.
The second reason is that people believe that "cartoons are for kids". This has been a problem for a long time. Back before the 1930's there were cartoons made for adults. Well at least as much as there could be in the 1920's. Betty Boop herself was riddled with sexual innuendo and people messing with authority. Unfortunately the Hayes code changed all of that. Cartoons since have been viewed as for children only, and American cartoons have never recovered.
We have made some progress. Cartoons such as Ren and Stimpy and the Simpsons proved that cartoons could be made for more mature audiences. Unfortunately, production companies got the wrong idea. Instead of understanding that cartoons could be made for adults if done correctly, they assumed that anything with
mature humor would sell, so a whole bunch of other very similar shows have appeared. Furthermore, when Anime came to America in the 80's, production companies screwed up yet again. Instead of understanding
why anime was popular, such as the more mature themes and a unique art style, they just assumed it was the way it was. Dozens of dubbing companies came along, yet not a single production company which tried to replicate the style or themes.
To be entirely fair, I am only talking about popular American animation. Independent animators have been making mature works for years, and a few of them such as Ralph Bakshi and more recently Don Hertzfeldt, have been received popular releases. Shows such as Avatar, The Legend of Korra, and the Boondocks, have been tackling mature themes, even if it is under the guise of being a children show/a humorous show. We're getting better, but we still have a long way to go.
If you want a way to increase the speed at which we will start to see mature animations in America, there is nothing better to do than to vote whit your wallet. Go buy Batman: TAS, go get Spawn: The Animation, or Don Hertzfeldt's volumes of animation. Show producers that you do care about mature themes in your animation, so they can show you that they understand by making them.