I
think it has to do with the fact that his sound is more influenced by Funk than it is Hip Hop. His lane is RnB judging by his beats. Quik's production reminds me of Roger and Zapp or Parliament. He makes laid back Funk music whereas his contemporaries such as Dre and Warren G know how to produce laid back Hip Hop tracks. I think Quik's natural element is 70's influenced RnB tracks. He's always been old-school.
I also think that his rapping voice was unmarketable. It was identical to Eazy's. He needed to come up with his own unique sound rather than using Eazy's.
We have to take into account that he is a literal nutjob
He likely could've never been a big producer because the moment he gets along with someone he's ready to fight them. How can you be a big producer if you get along with absolutely no one? He's clowned Dre and Suge in interviews and then praised them in the next interviews. Remember when he accused Andre 3K of telling everyone that he had AIDS?