Was never going to be easy to trade a guy with as much control over his destination as Kessel.
It was made doubly difficult because of the way he played himself out of town with temper tantrums, issues with the coach, etc. Nobody wanted the supposed "elite, PPG winger on a sweetheart deal" because, well, he's pretty garbage now. On pace for like 43pts and 16 goals.
He was instrumental in the back to back Cups, and we're all forever grateful, but the dude played himself out of town through his petulant outbursts and not giving a shit anymore. He got his name on the Cup, twice in fact, and he's got his money. He's been in full-on coast mode for years at this point.