When it comes to adult cartoons on the networks (not cable channels), Fox seems to do the most successful, while ABC, CBS, and NBC all seem to avoid adult cartoons.
Of course, once The Simpson’s began, ABC and CBS tried their hands at prime time cartoons and those failed. CBS tried Fish Police and Family Dog (which I saw when it was on the air), while ABC did Capitol Critters and the first season of The Critic (which was slightly successful, considering it got a second season… on Fox).
Yet Fox did not hop on the band wagon and do a new prime time cartoon (they already had The Simpson’s, but they could have tried a second adult cartoon sooner) for about six years with the second season of The Critic, and then it was seven or eight years before they did another successful long running prime time cartoon with King of the Hill.
After the success of South Park, many adult cartoons started to premier, especially around 1999, I guess this was the start of when Fox was really getting into more adult animation. But then with this era, most of the shows were on Fox, The WB, or UPN. The only adult cartoon since to come to any of the original “big three” networks is Clerks, which ABC mistreated.