Because owners don't usually tend to decide it's in their best interest to give out more of their money.
Major leaguers and their union have leverage, and that leverage could be used to help create a better situation for minor leaguers.
Some major leaguers have, upon reaching the majors, turned back to help the minor leaguers.
Many others don't look back, chalk it up to dues being paid, and that's that.
That comment you mention rubbed some people the wrong way, and that's OK, but I'll point out that I said I'm in favor of minor leaguers making more. What I'm not in favor of is some of the histrionics that have surrounded this debate. At last check, minor leaguers are not held against their will to play baseball. Those that do not like their situation are free to go seek more satisfying work. If we want to talk about finding more money for teachers, for police officers, for nurses, I'm ready to ride for the cause. Sign me up. But when playing baseball is given the same intensity in that kind of discussion, you start to lose me. The guys should get paid more, and there is no shortage of money going around in the game. On the list of things that matter the most to our society, this issue is far down the list.