He never played as a true left-mid, though. Marchisio was always inside, which is partly why Krasic started to stagnate, since the defense could just easily shift over to his side while abandoning the left.
Then when Marchisio played centrally, he failed to really impose himself on the game, just like he normally did while outside.
People always stress his supposed "technical ability", but even while playing further up the pitch in a role more intended for creative players, he rarely showed any of that technical ability.