I don't think Cancelos defensive ability is the main reason for him being sold, neither is it for us making money for other players.
Something happened between Allegri and Cancelo, diden't Cancelo like something on twitter about Allegri leaving or something? (Can't remember exactly what it was, but negative news for Allegri). Paratici and the rest agree with Allegri in his opinion about the situation with Cancelo?
Maby Juventus directors saw this whole incident as a sign that Cancelo has bad behaviour and attitude, this could be the main reason, just my own theory, but seems logical. Making money and his defensive ability as other reasons but not the main reason for him being sold.