well rickson gracie has the infamous 400-1 grappling record...plus his last name his gracie so that's why he is famous. marco ruas is apparantly a legend of the street and mad his claim to fame outside of MMA for the most part so that's why he is famous. as most people know i dislike tank abbot, but he is famous for having absolutely no MMA training and going in there and putting on entertaining fights. it was really the quality of the fights themselves, not of his opponents that made him famous. mark kerr beat randy couture in wrestling back at college and was a division I champion plus the 220 lbs freestyle champion. that coupled with the fact that he is a known weightlifter and ADCC combatant is the reason he is famous. Pretty much (besides abbot) these guys were known for their accolades outside of MMA and the fact that they competed in MMA later is the reason they are known in the MMA community. abbot was just exciting and he has a unique story.
also, a counter-argument could be that hoffman beat those fighter you mentioned before they were truly skilled. no one knew about herring or ricco when hoffman beat them, so maybe that's why he isn't as recognized. either way, you make good points and i agree that he should be more recognized then he is.
EDIT: and also, did hoffman beat a ninja like Ruas did...i didn't think so