After selling 2/3rds of holdings he doesn't have nearly the voting power. He'd likely not get support for a re-election, so easy out and save face by not running. Same as any incumbent politician that doesn't re-run for his office if he doesn't have the votes. Simple.
Most board seats are competitive. He has 6MM less votes than he did last time. Besides his background is more in development, not marketing and distribution. Major holders like Vanguard, Fidelity and Federated are going to want more Marketing and Production talent on the board.