He didn't influence hip-hop specifically.
So I guess the question is why is he acknowledged as an inspiration by some hip-hop artist.
This is how I see it. Many people in the hood had cable television by 1991. If you had cable, you probably watched MTV a lot, because they showed music videos. In a group like Onyx's case, they saw this really popular video with all these white kids just wilding out and jamming over and over on MTV, they related to the rebellious energy, and wanted to have something like that for people more like themselves. Then there are those kids in the hood who embraced skate culture, which pushed them even further towards hard rock & grunge. Plus the tour tees were pretty dope.
Onyx, Redman, DMX... it's clear they were in somewhat in tune with rock music in the late 80's. And maybe they were frustrated that signed hip-hop acts were forced to look and sound cool at all times, as opposed to being able to express themselves naturally.