Bob Arum gave Casimero the option of taking a reduced purse or he would move on to Jason Moloney.
https://www.badlefthook.com/2020/7/24/21337201/naoya-inoue-in-process-coming-united-states-casimero-fight-may-not-happen-boxing-news
Casimero's team did not accept the offer, they were probably thinking things would go back to normal soon and could get a much larger purse if they held out for a while. Despite trying to frame it as Inoue being scared, Sean Gibbons admitted that it was due to financial circumstances that they couldn’t reach an agreement.
This was then, but now Inoue has the opportunity to fight Casimero, The rivalry generates a lot of interest that it will convert to good revenue for both promotions, Inoue never mentioned Casimero when things got back to normal; he always opted for other fighters, and now that he is moving out of the division, he is still not showing interest in Casimero.
Casimero is a kind of fighter that loves to despise and make fun of fighters, he was actually selling a fight to the public, Inoue should have taken it back then, and now Casimero is not selling a possible fight anymore but directly insulting him.