Consolidation of hospitals into health systems and health systems into bigger health systems is part of the equation. Many CEOs who once led these formerly independent organizations don't want to work under circumstances of reduced responsibility and autonomy, and hey, they're pretty close to retirement anyway, so why not bait a hook and get out the golf clubs?
Another issue is that many large health systems that had previously worked as a holding company are making the transition toward being more of an operating company now.
What comes with that is less autonomy for former CEOs, and increasingly, if they're being asked to stay, their jobs are morphing to other titles and most importantly, to other responsibilities that are more in cohesion with the strategy of the whole—not their own local agenda. That's dispiriting to many CEOs.
Healthcare is a highly regulated industry, of course, so even a great leader can have a tough time against competition that is becoming national and global very quickly. She can be hamstrung by state regulation that isn't keeping up with the evolution of the industry while competitors operate without those constraints.