Decisions by judges got sketchy 10-15 years ago and it became unwatchable.
My father told me 60 years ago that boxing -- along with many other sports -- was rigged.
I’d say that the infinite numbers of “official” boxing organizations probably helped to end the romance as well. At any one time, there could have been too many “heavyweight champions” (as well as “champs” in all the other weigh classes) to count, most of which couldn’t compete with those of the halcyon days. This also contributed, in my opinion, to the sudden appearance of mountains of flab in the upper class.
No controlling authorities, and the paychecks for the best vs the up and coming. Imagine if someone starts a Consolidate Boxing Enterprise, and builds a way for training fighters, managing fighters, and promoting them? Like MMA?