Cause the level of his era was too poor. Every sport has to have some sort of starting point from when it starts to become advanced/professional enough to matter. In boxing, that's around the 1920s for me. Or in the most optimistic scenario, we can start off around the early 1900s. But even so, the level of the era must still be taken into account when assessing achievements.
No exactly. IMO boxing started to reach it's limits (human limits) in terms of skill in the 70s and it kinda peaked from the 80s to modern times. The HW division reached an all time high peak in the 90s. However, that doesn't mean that any champ/contender from that era is automatically better than a previous era contender/champion.
Counterpoint: Sullivan built the sport into the professionalized system that you recognize from the 1920s.
More to do that John L lacks fight footage. Outside of reports, what do we know about this guy?? One of the best of his era and is a great, but it is hard to judge something with no film on.
Peaked in the 70s and 80s? Based on what? Louis is literally picture perfect, by the book and Ali is fundamentally so broken he was befell by a mediocre Norton. If you were a coach you’d want a Louis and not an Ali as your product.
I am probably as staunch a defender of Sullivan as anybody her, but I could not rank him above Ali based on the available data. Sullivan's record is very impressive on paper, but we honestly don't know how good his opponents were, or that much about them generally. They were the best opponents available, for the most part, as far as I can tell. They were also the best of a chaotic era, where two rule sets existed in parallel, and you could argue that none of them was brilliant at either. It is possible that Sullivan benefited from that chaos. Many people are far too quick to assume that it was a weak era, but on balance, I think that it probably was.
John L has quite a few opponents listed as debutants on his MOQ record. However there was at least one of those opponents who was described in literature otherwise as having reasonable boxing experience by the time he fought John L - lots of holes in the history.