It's funny that you guys mention watching these after-school. Many of these cartoons were not technically "Saturday morning," but were syndicated and usually ran from 3-5PM. I saw a related video that explained the death of Saturday morning cartoons:
https://www.youtube.com/watch?v=3cpUDtW1cV8There were a bunch of causes, but the main ones were:
1) Cable networks like Disney Channel, Nickelodeon, and Cartoon Network having kids' shows available 24/7
2) New regulations in 1996 restricting advertising to 10 minutes per half-hour of (broadcast) kids' programming
3) Sports and/or news programming providing a bigger bang for the production dollar (especially considering point #2)
As a point of comparison, the contract that NBC signed with Notre Dame to broadcast college football cost them, on a weekly basis, about a third of what producing the cartoons in that slot did... and selling advertising for beer and cars is a lot more lucrative than advertising for toys and cereal.