I wonder if it's a realization that some of these cartoons are appealing to older kids, adults. Many cartoons have had inside jokes, for the adults that were watching with their kids. As noted, some cartoons, have been dark from the start.
The world is a "dark" place. No matter how peaceful, loving, and understanding some would like the world to be, the reality is often quite the opposite. Things are tough now, but it could be much worse. Many of us, no matter how tough things have been, have no idea of the stress that so many others have to go through. Life is not fair. Everyone does not win the blue ribbon. The good guys often loose. Dora the explorer is not how life works. We can't be a happy go lucky sponge living in a pineapple, making a living flipping burgers.
Or these dark plots could just be people trying to push the edge, be extreme, radical, cool...