I think there were several factors: Curt Flood broke the 'reserve clause;" MLBPA is probably the most powerful union in pro sports; agents, especially Scott Boras, are convincing players they should always shoot for the moon because often they will get it; owners in the big markets can pay big salaries and still make a profit. In his series on Baseball, Ken Burns has someone say that in the old days the average MLB player made 7 times the salary of the average fan. After Curt Flood and as of the making of that series, it was then 7 X 7 times (or 50 times as much as the average fan).
Years ago there was a very, very successful basketball coach at Butler. He and his wife Tracy were both from Indiana and were happy raising their kids in Indianapolis--plus she was/is a lawyer. His salary was $1M/year, which was all he needed but well under his probable value at a major program, several of which came calling while he was at Butler. I would have bet he would never leave Butler, but in fact his wife negotiated the contract with the Celtics for six years and $22M. I also would have bet you can't transfer great college coaching skill into success at the NBA level. My guess is that the whole Brad Stevens family is thriving in Boston as I write this. My point, I guess, is that people are unpredictable.