I think the reasons Cann hasn't argued a call are two-fold: relative youth & inexperience as a coach, a college coach, & let alone a HC. I'm not implying he doesn't know how or when to argue; or that he doesn't understand the potential motivational factor doing so can produce.
IMHO, he is simply (& possibly wisely) biding his time & paying his dues. This year it goes well with his mantra of this pitch, this at bat, this batter, this inning, this game...control what you can control, etc. He seems a player's coach, & he will inevitably vehemently stand up for them.
Plus, he probably doesn't want to be viewed like "Pulmonary."