Right now, Jerry DiPoto (Seattle Mariners) is the only current GM to have played Major League Baseball. The position used to be held by former players all the time.
Now several GMs, including Mike Hazel, Dave Forst and Ross Atkins, played minor league baseball.
The dedication to analytical baseball might be one reason so few players hold the position, but skyrocketing MLB salaries could be a factor, too. Maybe as many former players may not need to work anymore as there used to be.