Even though everyone was waiting for the first rank Sun Mu to accept the challenge of Xu Ming, it didn't happen.
One month passed, and there was no information about Sun Mu accepting the challenge.
Even though everyone else had to fight at least one ranking battle every month, there was no restriction like that for the 1st rank.
It was said that only someone from the top 3 was allowed to challenge the Rank one, and accepting the challenge entirely depended on the 1st ranked person. They only had to fight once every year instead of once every month.
No one had any information as to why Sun Mu wasn't accepting the challenges. It was said that he was only focused on the Core Sect trials to become the Core Sect disciple, and he didn't want to waste time fighting anyone unless he had to.
That's why, even when Xu Ming beat his brother Sun Zhen, he never came for revenge. It was clear that he wasn't interested in revenge; otherwise, he would have accepted the challenge.