My thinking is that the ground opening demonstrated that Korach’s actions were wrong, e.g., rebelling against Moshe’s authority, but didn’t conclusively demonstrate that it was Hashem who chose Aharon to be kohen gadol rather than Moshe. The budding staff conclusively demonstrated that Hashem chose Aharon.
I also agree with WolfishMusings that the staff served as a permanent reminder that Hashem set the laws of priestly succession to ward off future disputes.