Well we need to put context in order,Invader Zim was a show with a unique animation style even today as well as extensive use of CGI in a time when putting them in cartoons was still something that exited audiences. This made the IZ episodes have high quality but expensive formats. This played a part in why Nick executives felt the show's ratings weren't making up for it cost and canceled it prematurely despite its Imenses fan base. This was generally seen as a bad move in hindsight and made many fans angry with little benefit to nick.
SU by contrast has nice cheap, easy to draw,easy to pump out Calarts style where half the characters are shaded blobs. Great for the networks budget and demands.
One must also take into account that IZ time had (let's face it) better completion with iconic shows like Justice league,KND,Grim Adventures of Billy and Mandy,Ed,Edd and Eddy,My life as a teenage robot,Danny phantom,Jimmy Neutron and of course Teen Titans,Sponge Bob and Fairy Odd parents when they were good
By contrast what is SU completion?
Clarence,Teen titans go,Regular show,the bloated renements of SpongeBob and Fairy odd parents?Sure we got Gumball and Adventure time but by comparison IZ had to fight to survive against lean competitors while SU gets the Network to spoon feed it and change its diapers.
And lastly we get to niche,IZ was edgy,literally with straight lines every where and the authors dark vibe all over the place. This meant it had a more limited view base as it appealed more to teens and boys while SU has a mishmash of generally "family friendly" themes with drama and family ,friendship and love as its cores. This allowed to have a broader audience than Invader Zim but this also got it an audience that did not exist in IZ times. The Tumblr,we can't fail to acknowledge that no show has gotten tumblers fancy like SU and as such the whole site and those who dwell there have become SU greatest oviators.