I think you should put "educational" in scare quotes. Some of the crap that gets the E/I tag (aka HEY LOOK WE'RE SHOWING THAT EDUCATIONAL STUFF, SEE?) is hard to call either educational or instructional. Especially those "teens do teen things at school" sitcom shows (well "sit" anyhow, not much comedy). Mostly it's just three hours of "wildlife" (aka look at these random cute and token non-cute animals) shows. Which are also shoehorned onto the weather sub-channels on Sunday mornings. One of the channels here plays something called "Tomorrow Today" which seems to be some sort of Australian science snippets show that I was unable to trace the origins of. (and it's produced in 4:3 too) But at least it's showing actual science-y stuff, and not teens-being-teens. (TOOOOOTALLY TEEN!)
Another part of the downfall was when it was outlawed to advertise toys along with the show that those toys were based on, when a bunch of soccer moms got in a snit about that. That certainly reduced the interest in creating more than a few shows. Meanwhile, in Japan, that certainly hasn't hurt the chirlrrrrrren.
And we must also not forget the after-school block that was big in the '90s. I guess those got killed off by courtroom shows, adult talk shows, and celebritard gossip shows, because that's what I see when looking back to last Friday afternoon in my MythTV schedule (which was harvested from the actual OTA guide info).
But really, I'm going to have to say that pervasive cable TV and cable-only networks in the US was what really happened. Why get up early Saturday morning (or rush home after school) for your toons, when there are multiple channels showing them 24/7?