Dismissed for his loss to Corbett when he was old, rusty and pickled. Everyone ignores all anecdotal evidence- and his record- for greatness. Sure he was a first *******. But so was Ty Cobb. Boxing legacy and ability? By all evidence he was a phenom and should still be rated a top-10 all time great.
I think people leave him out as the last of the "pre-modern" champions, though he's actually acknowledged as the first Queensberry champion. Interesting thing about Sullivan, in the age of epic 2-hour 50+ round battles, he was knocking lots of guys out in 1, 2, 3 rounds or so. London Prize Ring rules or Queensberry, either way he was getting rid early. Maybe the opponents were blacksmiths and drunks but other champion fighters of the era were slugging it out for hours with their opponents , what Sullivan was doing was unheard of.
I think in part because Sullivan is commonly, but incorrectly, thought of as a London rules specialist. I think Sulivan would have been at his best in Queensbury fights where KO's were allowed. Unfortunately for Sullivan, a lot of his Queensbury fights were under close legal scrutiny, and he couldn't go for KO's. Another factor in thinking of Sullivan as a London Specialist is that in most recent popular books covering the lineage of the heavyweight championship the Sullivan fights that get the most coverage are Ryan I, Mitchell (in France), Kilrain and Corbett. This gives an impression that Sullivan mostly fought London then, at the end, fought Queensbury against Corbett and was out of his element. Also, Sullivan was dominant enough to remain champion many years after his prime. Most observers who cite Sullivan as an ATG note that they are refering to the Sullivan of the first half of the 1880's. That Sullivan was quick, active, two-fisted, hard to hit well, and was (by 1882) being cited as a straight hitter. In the second half of the 1880's these qualities gradually abated. Also, after breaking his left arm, he became over-reliant on his right. I wonder, as well, whether the film 'Gentleman Jim' might have helped fix the image of a crudely swinging older Sullivan into the publics mind.
I think the problem is that his oponents are not known and the historical significance of his wins is therfore lost.
The problem is he fought in an era where neither Queensberry Rules or LPR were generally used. Sullivan fought a hybrid of LPR with gloves most of the time, and wasted a lot of his talent because of his drink problem and a lack of the basics. Which to a point showed you how good he could of been.... He normally fought four round exibitions with gloves, but with throwing and wrestling allowed. And most of the time Sullivan was not in shape.
The farther back in time you go, the easier it is to dismiss fighters compared to the latter crop, who there's film of, among other things. One of the reasons I think.