Because adult swim was the first, so Nick never came up with the idea before hand, and most other children's networks don't bother.Now that you mention it, it is a wonder more "childrens" channels haven't added "Adult Swim"-style late-night blocks. Though I've never been interested in Invader Zim (it began at a time when I wasn't too interested in new Nickelodeon progamming, though I still watched the channel a lot... And I think I did regularly watch new Cartoon Network programming at the time). Maybe Nick at Nite should have devoted an hour a night (maybe the 11PM time slot) to to original programming for older audiences. Most of the shows on Nick at Nite are appropriate for kids. And as I mentioned in the first post, Nick at Nite has had original television series over the years, even during the "Classic TV" years.
I give the digital channel qubo a LOT of credit for its Night Owl programming block. At midnight up until 5 in the morning it airs hours of He-Man, She-ra, Filmation Ghostbusters, Bravestarr, and the only non-Filmation show, Spliced. Not even adult shows, mind you (though Spliced WRONGFULLY has a TV PG rating), but shows that would attract an older, more nostalgic audience.
Now It's not that I hate Nick at Nite... but let's face it. The idea of a 24/7 kid's network never took off until recently, where we have stuff like Nicktoons. It was a great place for old sitcoms, sure.... now it's a dumping ground of current stuff that even syndication doesn't want to air anymore. Still, can someone PLEASE tell me the appeal of According to Jim? One of my syndicated channels still runs that abhorrent piece of dung. i'd assume Nick airs it too.