Eshval's reasons for leaving are because they were abducted by alians and they put a probe in place much like Cartman's, Eshval can no longer sit in a chair long enough to play CF due to the giant Sattelite dish that deploys itself periodically from thier orfices.
In reality I dont want to know why they left. I'm a firm believer that what happens behind the scenes by leadership in scenarios that have a potential negative impact on morale/unity of the leadership group should be kept behind closed doors. And if such was the case I respect Eshval's decision to do so. For all we know they could just be completely burned out on the game from putting so much effort into it and they decided it was time to dedicate more of thier free time to doing the RL things. Or they could have just gotten a smoking hot girlfriend that keeps them too busy to play. Who knows.