The answer boils down to about two points: Market size and collective negotiations.

Back in the day, sports athletes were considered amateurs and were more motivated by passion to engage in a hobby more than working a primary job for earning income. As time went on through the 30s and 40s, competition and swelling audience sizes encouraged the development of a more professional caste of athletes who were naturally a bit better paid than their amateur predecessors, but we’re still not close to the modern multimillion dollar contract. By the 1950s broadcasting emerged as the big game changer, catapulted by effective collective action. With broadcasting the audience size increased exponentially, and earning of the overall sport. As owners and broadcasters made more and more, players increasingly organized to bargain for better pay. Through the 1970s and onward, as player organizations and unions became powerful and ubiquitous, and rating soared to higher and higher levels, players’ salaries ballooned.

Keep in mind, though, players’ salaries vary wildly even with the same league or even the same team.

https://www.google.com/amp/s/www.firmex.com/thedealroom/when-did-athletes-start-getting-rich/amp/

https://operations.nfl.com/the-players/evolution-of-the-nfl-player/

http://mentalfloss.com/article/84792/11-things-you-might-not-know-about-athlete-salaries