Colin Hart in The Sun, who is a good judge of boxing and speaks a lot of sense, says it is the correct move by WK as he thinks that AJ now has the measure of him and would beat him a lot quicker in a rematch. So why should he risk getting beaten up.
I think that he realized he was well beaten and didnt have anything different to give if the rematch took place. The problem is that there is nobody anywhere near capable of beating AJ at the moment so he will have trouble getting meaningfull fights