We know the mandatory retirement age will likely go up again "soon", probably to 68 like in Japan and then further on to 70.
So if your life goals include the majors, the incentive is to do whatever it takes to move now (finish that 4 year degree, etc) -- or risk getting stuck in the regionals for a many extra years, which is what happened the last time the retirement age was raised.
On the flip side, if you take a pay cut to go to the majors, there is a risk of a long wait before upgrade there if the retirement age keeps getting bumped up all the way to 70. Those who are now nearing retirement will have huge financial incentives to stay on a few more years, with the QOL of having the most seniority and earning money at the top of the pay scale, while everyone else waits below.
Lastly, there are many very Sr. Captains who are comfortable at the regionals but who may not have the talent or determination or education to successfully make the transition to the majors, even if they want to. Some may not have spotless PRIA records, etc. So they're not going to risk losing everything just to go to the majors, and will just stick it out at the regionals.