He is just a promising youngster its not like he is Messi or something
Not gonna dispute that last part, but he lacks mental toughness to become a captain. His inconsistency issues mostly arise from this. He is a follower, loyal soldier, but captains are built from something else. See Vidal, Buffon, Chielini, players that wore an armband before like Del Piero, Conte, etc... Not quite the same. His drifting out of the game for large periods of time is making him far from ideal candidate.
I think his inconsistency last season was mainly due to having periods of limited game time and that injury he had for quite a while... I think Marchisio is very consistent in his perfomances, always fills out his role in the team. It's right though, that he only sometimes add those extraordinary that at least one player needs to deliver each game. Pogba more often makes that magic pass or scores a goal out of nothing to break down a tough defense, but so say Marchisio often plays badly is wrong imo.