We let Harper go because he had become a distraction, and because Rizzo and crew (correctly) calculated that Eaton & Corbin > Harper. Remember, had we signed Harper there was no money / cap space to sign Corbin, and we would have had to have traded Eaton to make room for Harper with Robles in CF and Soto in LF. Eaton and Corbin were instrumental to the 2019 WS championship.
My guess is the final straw for Harper was when he tweeted a picture of himself, Robles, and Soto as the OF of the future or something like that, while Eaton was still on the team.