It doesnt indicate any such thing. The vast majority of professional fights that took place before World War 2 are not recorded on Boxrec.
Excellent point. And if the U.S. hadn't developed a robust amateur boxing scene, we'd almost certainly have way more of the low-quality, small-scale "pro" cards between schoolboys and novice boxers that flourished around the country earlier in the 20th century.
Do people imagine that the Archangel blew a golden trumpet, and then announced that every professional boxing match in history would be listed on Boxrec? A fight only gets listed on Boxrec if a modern researcher discovers a contemporary source relating to it. Unless one of the participants is a fighter well known today, they are not even looking for it!
Boxing isn't even in the top ten most popular sports in the world. But either way boxing is boxing. Always has been always will be. Its been around much longer than all the measurable sports and team sports. It's fruitless to compare the evolution of these other sports to boxing or any other combat sport that's been around since ancient times.
I think the point was to show that in 2010 with three times the world's population of the 1950s, numbers of recorded fights dropped bellow 1950 figures..