There are two main suspected reasons for the delay. One is the ratings disappointment of Hey Arnold!: The Jungle Movie. The first of the current wave of Nicktoon revivals to be completed and the only one to actually air, The Jungle Movie was mostly well-received by those who watched it, but the network was apparently disappointed that not as many were watching it as hoped.
An average of 1.63 million viewers isn't a disastrous rating as cable viewership has declined overall, but it was poor enough that Nick canceled all scheduled repeats of the movie.
It's hard to get kids to watch continuations of old shows they never really had the chance to get into, and it's hard getting the attention of adults who might reminisce about Nick's good old days but never turn to the channel today.
Rumor is that Nick is trying to navigate around these obstacles by eventually releasing Static Cling, as well as Into the Florpus, straight to streaming. Which streaming service that might be is uncertain, however. Nick licenses some of its shows to Hulu and Amazon Prime, while VRV hosts the NickSplat streaming channel, which hosts the original Rocko and would be a branding fit in a more niche market.
Then there's the recently announced deal between Nickelodeon and Netflix. Netflix has already premiered the long-delayed Nicktoon Pinky Malinky and started development on a live-action adaptation of Avatar: The Last Airbender. Now the two companies' partnership has expanded to include movie versions of The Loud House (previously scheduled for a 2020 theatrical release) and Rise of the Teenage Mutant Ninja Turtles. Could the Rocko special and the ZIM movie join them?
The other big factor that might be complicating the release of Rocko's Modern Life: Static Cling is Nickelodeon's massive changing of the guard over the past few years. Cyma Zarghami stepped down as President of Nickelodeon in June of 2018 after 30 years at the network. Brian Robbins filled the position four months later. As Nick's dominance in the ratings has fallen drastically, Viacom corporate management has reportedly pushed for more shake-ups.
These corporate changeovers can often lead to projects spearheaded by previous management falling through the cracks or getting short shrift. This is Joe Murray's big fear for Static Cling: that the new executives won't give his movie the same support. Static Cling would not be the only Nicktoon project to face such difficulties recently: Pinky Malinky was delayed for years before being dumped on Netflix, and another long in-the-works Nicktoon Glitch Techs is also reportedly facing production difficulties.