Davis gave his time to a trash organization that doesn't know how to contend. They gave money to a guard who isn't any good because his wife was sick and it was a touching story. They are always a step behind. He decided he wanted out.
Boston assets are vastly overhyped. Those assets are not nearly as enticing with Kings/Nets improvement. Their players? What do you want? Jayson Tatum? Terry Rozier? Jaylen Brown? All the crap the Lakers get for not having "talent"--look what their discarded players have done--Russell was an All Star and helped get Nets to the Playoffs, and Randle put back to back strong seasons up. Meanwhile the last team to deal with Boston was Cleveland and where is Jae Crowder and Isaiah Thomas? On different teams and didn't do anything for Cavs and not doing much for Jazz and Nuggets. Boston has Woj sucking off Danny Ainge and all he does. Irving is gone after this year....and you trade away Tatum/Brown to get Davis so what do you have now?
If Durant and Irving go to the Knicks--and Knicks were to get #1 pick, they could trade that, Knox, etc...for AD and it's a far better deal then anything the Celtics could offer. Would you rather have Zion Williamson/Kevin Knox or Boston's offer? Davis would sign an extension with Knicks--if he goes to Boston, he'll be 1 and done and you gave up a bunch to get rental for 2nd big trade you've made with what to show for it?