Because Xun Yu has demonstrated that he opposed Cao Cao's plan for a Wei dynasty. By brewing revolts and legitimizing Shu and Wu, the things that has been brewing for the last few chapters now.
That's a decent tactic, forcing Cao Cao to keep his reputation until he dies, lest himself become another public enemy like Dong Zhuo. Unfortunately it could never last long, as his successor would have no qualm replacing Imperial Han. Is Xun Yu making this desperate move while hoping future generation will solve the core problem for him?
After Liu Bang became the first Han emperor, Zhang Liang, one of his most important advisers, retired young. That was the correct decision, because Liu Bang then purged everyone who supported him.