I think it's the culture we're creating, one that works hard and wants to find an easy way to get their kid to sit still. So they look for the easy way out: the television.
Otherwise the child might be running around town causing trouble, which causes worry for the parent.
Parents are having to get more jobs, giving them less energy to care for their children at home. Parents are wanting to dump their kids off at school for a break, making our education system naturally more unrefined.
And really, it does also depend on where you're from. If you live in a big city, it's usually best to keep your kids in-doors. It's not like rural areas where there are lots of places to play without having to worry about creeps and gangs. City streets are just dangerous places to leave kids, so keeping them in-doors requires something to keep them busy. Once again we go back to the TV, with its cartoons to act as a glorified supplemental toy.