These are Morata's stats this season...
Asking him to score 20 league goals is nothing crazy when he scored 15 in 1300 mins (literally 1 goal for every 90 mins of league football). So we're not expecting him to make any great "leap", just to simply adjust to the league and deliver same form as he did for Real last season. Even an inferior form (in terms of minutes to goal) would make him a success. Ibra had a goal every 143 mins in the league (which is literally 60% worse off than Morata) and yet we are talking about how he was a big success.
On the subject of why he doesn't start ahead of Benzema, it could be simply because of dressing room dynamics, hierarchy or managerial/presidential personal preference. All that stuff happens a lot in Madrid. I've not seen any evidence of him failing to link with team-mates when he's played.