Precisely. A cult following alone doesn't guarantee a series' success. It needs to be commercially successful in order to survive. If a show on Nickelodeon gains a fan following among Gen X, Gen Y, Gen Z, Zoomers or Becomers or whatever, that's nice and all, but ultimately that doesn't mean much to the folks in charge of Nick because they're not the channel's target demographic. Twenty somethings aren't the ones buying the toys, playing the games and downloading the apps, and it's considered to be a waste of money to run ads targeted to kids if the kids aren't watching.Think this question gets asked at least once a year.
Basically, it boils down to advertising. SpongeBob is a ratings driver, allowing Nick to charge more for ads, which helps feed the network. Unfortunately, this means some shows fall by the wayside. If shows don't perform well, Nick won't be able to charge much for ads.
Although some shows appear to do well, they may not be doing well in Nick's key demo, which advertisers want to advertise to.