The reason why people are calling for his head this season and not last year is all about form.
Take it simply that in Zlatan's first year, Del Piero was not so consistent with his performances and the rise of Zlatan was just what everyone needed so everyone loved him. That seemed fair enough to all of us since we want players out there who are peforming and who puts the ball into the back of the net rather than who we fans want to be playing. When people last year were calling for Del Piero to be started, I didn't think it was fair because Zlatan was having a wonderful season that we needed to exploit him. On the other hand, Del Piero did have his highs and lows, his high was definitely the Scudetto showdown with Milan which he won singlehandedly for us with his imagination. But despite that, he had a very quiet season in deed.
Performance is all that counts.
This year though, Capello started the season thinking that Zlatan would start-off in the same foot as last year. But after a couple of games, it seemed forever for Zlatan to pick-up his game and get back in the groove. Del Piero on the other hand, gave match-winning peformances when called upon.
People started realizing that Del Piero was more effective and not only because of the statistics. But the fact that Zlatan was still getting more matches than an effective Del Piero, who rippled the net more often than the Swede the whole season, began jolting the people and asking for 'Il Capitano to start more matches. But no, Zlatan still got the nod more often. Any neutral fan would've questioned Capello's decisions and this is why I believe people are calling for Zlatan's head.
Why Capello doesn't use the same treatment Del Piero get when Pinturicchio was off-form is something that puzzles a lot of people. I thought he played people who were on-form? And if this is the case, the Del Piero definitely deserves the playing time he's getting now, and Zlatan on the bench seems more justifiable.