I wonder about Sang Zan. He was also trapped in a pillar, and there were the ghosts of his tribe as well, everyone who'd turned against him and Ge Lan. Maybe they couldn't reach him, so he didn't constantly get it all reinforced? I don't think we know ...

Part of it could also be just the timescale? Sang Zan was empillared for 300 years IIRC (or 100? anyway), while Ye Zun was in there for 10 000, which is like over 30× as long. Lots more time to go mad in 10k years!