Chandler's contract was up, Pitbull's isnt.
Bellator and coker seemed over Chandler and his high salary esp once he lost the title. The writing was on the wall with things Coker was saying and fact they let him become a free agent.
Chandler then also wanted to go to UFC. The covid stuff might have factored in too cause PFL isnt doing events till next year and you still arent seeing MM, Eddie, Sage fighting in One so do you want to sign with them and then travel issues come up and you dont fight for a year or more? So with Bellator not being as interested and other options not really looking great the UFC was the best option. Maybe thats where he would have went all along but if times were normal he might have went elsewhere.
As for Pitbull he's still a Bellator guy, him, Lima and Chandler before now all reup with Bellator anytime their contracts were up. They seem happy there, treated well and paid well.
It would be up to Pitbull when his contract is up to test the market to see if UFC would pay him or not. You cant just steal someone, they have to be free to leave and actually want to leave and I'm not sure either of them do.