I think a lot of it has to do with two things, that lots of dark stuff is superficial and not well done, and it's an overall attitude problem with people. For the first point, you get people who think that just throwing in sex, drugs, and violence makes it mature, therefore better than the stuff for kids. People don't realize that there has to be story and context with those themes for it to be actually be considered mature. Throwing it in for the sake of it isn't mature. Throwing it in their to make a point, or if it's a natural thing in the story to show consequence, that's mature adult themes.
The second point I feel is the more important one, or at least it directly involves the consumer. You get people who are afraid to be looked down upon for what they like, and there is this perception that if something is dark and gritty, it's mature and people won't look down on you, this can be seen with games to a degree. If someone watches a cartoony show, that on the surface doesn't seem to deal with "mature" things, then it's not mature, and only kids watch that. As an example, you will get people who call the Lego movie dumb not because of well thought out reasons, but because it's lack of blood and other things show's it's not mature, but then might point out to Prometheus, and say that's mature because it had death, blood, and other stuff.
Overall, it comes down to the execution of the media for if it truly is mature or not. Avatar the Last Airbender and the Lego Movie are mature because it shows certain subject matters in a subtle but good way that older audiences can see. There are media that is dark and gritty, but can still get across a mature theme that deals with adult situations, and is not mature because of the fact it has blood, or everything is dark.