They say that Nickelodeon was looking for a cartoon aimed at their Older Demographic, and so in 2001, Invader ZIM was born.
This pre-dated Cartoon Network's Adult Swim by half a year, so you really couldn't say the market was clamoring for an adult-oriented children's cartoon (as opposed to an adult-oriented adult cartoon like the film 'Heavy Metal' or anything you'd find on MTV back in the day or Comedy Central today). In fact, given the time frame, it's difficult to see why they'd want to market to adults at all, since Nick has always prided itself on being the only network for Kids.
Marketing cartoons to adults is stupid anyway. If an adult latches on to a child's cartoon, it's almost certainly by accident and in spite of the childishness of the show itself. See: Spongebob Squarepants. EVERYONE freakin' loves that guy, but it's not like Nickelodeon's airing it at 11pm so adults have an alternative to Daily Show repeats. They air it in prime kid time and the adults just have to adjust or catch it on one of its millions and millions of re-runs.
That having been said, I'm kind of surprised Invader ZIM didn't make it as a kid's show; it strikes me as something I would've fawned over as a 10 year old. I mean, for Christ's sake, Ren & Stimpy was at the core of their Saturday Morning Lineup! Looking back at some of the themes and gags, you're telling me a 10 year old mind can fully comprehend what's really going on in 'Space Madness'?
Invader ZIM reminds me in a lot of ways to Ren & Stimpy. You've got the diminutive little asshole alien bound for world domination through antisocial tendencies and a general distaste for the pit-i-ful HUmans; and you've got Ren, the diminutive little asshole chihuahua bound for fame and glory and awesome pecs. Zim has his lovable, good-natured foil in G.I.R. and Ren has his Stimpson J. Cat. The social dynamic between the main characters and those surrounding them are fairly similar as well, with the bulk of the Earthly population being bumbling morons who are merely fodder for the main characters' whims.
The obvious differences lie in animation style (ZIM tending toward that slick Anime style without being too over the top; think Boondocks, since both creators started out in comics) and overall storyline. That is, Zim actually HAS a storyline, while R&S lean towards those classic Bugs Bunny types of one-offs, only more perverse and with far more boogers.
The worst thing you can do to a show is shift its timeslot; especially on cable and ESPECIALLY when you're a children's network and kids have the memory retention of a squirrel. If the fans can't find the show, then they won't watch; and if potential fans can't find the show, they'll never know it existed until its 27 episodes are packaged on a DVD set seven and a half years after the fact.
But, some things just weren't meant to be. I mean, it's not like ZIM had a block of popular like-minded cartoons coming out with it. It was on an island out there while Ren & Stimpy had Rugrats and Doug all joining the Nicktoons fray at the same time in the early 90s.
I will say this, the best children's cartoons are those that appeal greatly to adults. I felt that way when I was 10 and it's still true. Invader ZIM followed a long line, from Ren & Stimpy and Rocco's Modern Life and Animaniacs and Eek The Cat and Life With Louie and The Real Ghostbusters and Teenage Mutant Ninja Turtles and The Transformers and anything made by Warner Brothers before the 1960s. You don't NEED to be an adult to enjoy those shows, which I did, but in being an adult, you can look back on these shows and realize they did something special while all the G.I. Joes and Alvin & The Chipmunks and any of the myriad of bastard Walt Disney spinoffs failed miserably.
A classic cartoon will always withstand the passage of time and keep well-preserved long after childhood whimsy has passed you by.