The whole story is in the spoiler tag for length, but the tl;dr is KeMonito has wanted to retire for a while and was convinced by CMLL to keep sticking around until the pandemic hit and they left him high and dry. CMLL cheaping out on paying their talent is fairly common, but he was such a well known icon it's a little surprising they did it to him.
I suppose CMLL just believes they can stick someone else in the monkey suit and no one would notice, but KeMonito was small even by mini standards and anyone else in the suit would be immediately noticeable by anyone even slightly paying attention.