Part of it for us was the 'zoning out'. We had Netflix so no commercials, but he'd sit in his chair for an hour or more and just suck his thumb and zone out. Afterwards, he just wanted more. I guess that is a lot of the appeal of TV, just escaping from the every day of what's around us.
There is a ton of crass children's programming, but I wouldn't put Dora/Diego and other Nick Jr. content in that category. It has some educational content, and it doesn't even have commercial interruptions when it runs on Nick Jr. The worst I could say is that it grooms kids for the more crass/commercial stuff that comes when they migrate from Nick Jr to Nick and Cartoon Network and Disney Channel.