1. Boxing fans want to see the best fighting the best. This happens less frequently now. They marinate fights. They duck fights. They make the fans wait a few years for a fight they want to see right now.
2. Some fights have not delivered to the hype and it's the sports fault. They have become boring. Fool me once it's your fault, fool me twice it's my fault. The world was excited about Mayweather vs Paquiao. Even granmothers watched that fight. Many hardcore fans thought it was an interesting 8-4 type of schooling. But the casual fans saw it better, they saw a man hug another man several times each round to avoid a fight. Memes were created about this. Favoritism and lack of rules enforcing by referees don't help. Deduct points to those who break the rule of excessive clinching.
Fighters selecting the opponent gloves, etc. It has become a joke. A-sides being getting away with a failed drug test.
3. Robberies. A-side getting help on the scorecards has become so frequent that it has also hunted casuals away. I don't even want to give examples here.
4. Too many divisions and organizations. There was a time in which boxing had 8 divisions or so? maybe 10? and two recognized world title organizations? now we have four and something like 16 or 18 weight classes? it's hard to recognize who are the real champions now. There are many.
There are probably some other reasons. They are treating the boxing fan as their bitch. Do you think this wouldn't bring any consequence?
Are you surprised that casuals are watching youtubers?
get rid of the adalaide byrd's of the sport, bad cards = goodbye, maybe its impossible to get neutral officials, they know who they want to win when it comes to the big guys, clear route to top guy would also be good
UFC you are a lot more likely to see the best fight the best and you are a lot less likely to see a robbery because so many of those fights dont reach the decision point.
Just in terms of the USA, boxing has always been corrupt, promoters have always been greedy and robberies have always happened. Maybe sports fans just have a lot more choices in how they spend their down time.