Part of that is due to the fact that more serious cartoons never get the marketing budget the more child-oriented fare does. Nickelodeon made sure everyone knew Spongebob Squarepants existed; what did Disney do for Gargoyles?
Back in the Hayday of the Disney Afternoon? Quite a bit actually (alongside
Ducktales, Chip and Dale: Rescue Rangers,
Gummi Bears,
Talespin, and a few others). The real problem is that it was going up against cartoon heavyweights like
Chip and Dale: Rescue Rangers, Talespin, and
Ducktales within that block.
Another 'serious' cartoon that showed during the later years of the Disney Afternoon was
Filmore, which parodied the older cop shows while laying it serious on the seriousness. That show had to compete with
Ducktales (which was still having re-runs at the time) and
Recess.
That's where the big merchandise sales are, supposedly, so from a straight business perspective that does make some sense. On the flip side, it's equally valid to say that a show that caters to a larger age bracket, like something that's PG-13+, can make you even more merchandise money because the scope of said merchandise has just broadened considerably.
That isn't the case, I'm afraid. The sad reality, from what I can tell, is that the higher the minimum age bracket, the
worse your RoI is when it comes to animation (and, surprisingly, movies as well).
People forget Disney's
biggest bomb that was The Black Cauldron, which in today's rating system would be PG-13 (PG-13 didn't exist in 1985, The Black Cauldron was likely one of the instigators of that rating). When I say it bombed, I mean it
cratered. As in 'it didn't even make
half its budget' bad (it made 21.4 million at the box office and
that with a budget of 44 million). It was so bad that
Disney swore off anything much above PG for decades outside its Indie studio Touchstone.