Risk aversion. He knows he got lucky against Joshua and is trying his hardest to stick around for a 3rd Joshua fight. Has overvalued himslelf and the payday to fight someone he knows he can beat is too low.
He might be awaiting his rubber match with Anthony Joshua, who he battered and KO'd in their first meeting.
Ruiz was more focused on cleaning out the fridge than the division. I think he should do a trilogy fight with Joshua as they make for an interesting matchup style wise. Both are risk adverse so why not?
He doesn't care about the sport, and he has enough money to wait and possibly be picked by a cash cow to lose to. Why lose health?