I think a more interesting question is ‘why is boxing so weak in MMA, relative to other combat sports’....and even that’s not a particularly interesting question.
The answer is surely money. If you can box at a decent level, then in most cases it makes sense to stick with boxing. Same with Muay Thai or kickboxing - both have avenues to make money. There’s a reason we see world class grapplers go into MMA but very rarely (if ever) see a world class striker do the same.
Where do talented grapplers like Romero or Maia really go to make money after they’ve been successful in wrestling/BJJ? The obvious answer is MMA.
The notion that boxing doesn’t work in MMA is completely false though. Like any other art, it just needs adapted. Wrestlers don’t use a conventional wrestling stance in MMA, Jits guys don’t use a jits stance, Muay Thai guys don’t use a traditional Thai stance.....obviously any boxer is going to need to adapt too. They absolutely could and do make dangerous fighters though - punching someone in the head is and always will be the quickest and most high percentage way of knocking someone out.
And JDS is absolutely NOT an elite level boxer.