For a series of reasons:
A. Poor infrastructure quality: the vast majority of the stadiums in Italy are owned by the municipalities. Italian municipalities are highly corrupted entities / do not directly benefit from increased attendances to the stadium (i.e. they get a fixed licensing fee from the Teams, regardless of attendances): for these reasons the stadiums have not been either maintained or renovated in decades and, as we all know, people in 2015 do not accept to go to the same stadium that was considered “state of the art” in 1989 and has been left untouched and decaying since then. The problem is exacerbated by the fact that Municipalities gain money by allowing the teams to play onlyin the Municipality’s stadiums (through a sort of licensing fee which is negotiated on a yearly basis): for this reason, not only they leave their own stadiums to decay, but they also have no interest to allow the Teams to build their own stadiums as the Municipalities would lose an important revenue stream for their finances (e.g. just look at the Naples Municipality and its now-in-ruins San Paolo Stadium: the Mayor of Naples refuses to either renovate it (blaming the lack of budget) or to authorise the Team to build its own infrastructure, otherwise they will lose the licensing fee: this is a disaster for such an ambitious and successful team). Please note that most of the stadiums in Italy have been built/re-built in the late 80s for the Italy 1990 World Championship. These stadiums are architecturally terrible, built at the end of the disastrously corrupted 80s, and they try to do too many things at once without doing anything right. They are also enormous (many of them can seat more than 80k supporters), the supporters are very far from the pitch so the players look very tiny, there are not even comfortable seats. On the opposite, the 40k seats Juventus Stadium in Turin has been completed 3 years ago, it is considered “state of the art” by 2015 standards and is consistently sold-out (even notwithstanding its higher-than-average prices).
B. Bad connections from city centers: the public transports in Italy are pathetic: in order to reach most of the stadiums in Italy you need to take a car or a scooter. This makes things bad on the way tothe stadium but things are utterly deadly on your way backhome at the end of the match. The roads around the stadiums connecting it to the city centers are simply unsuitable for thousands and thousands of cars flooding back to the city so you can easily remain stuck at the stadium for more than 1 or 2 hours after the game has ended. This is unacceptable for modern standards but the Municipalities have simply no interest in fixing this.
C. Violence by Organised Supporter Groups: in Italy (from North to South, East to West) it is still completely normal for a (small but influential) fringe of supporters to go the stadium not to watch a football game but to have an excuse to make riots, destroy everything and beat (sometimes even kill!) the opponents’ supporters. Roma, Genoa, Verona & Lazio supporters are among the most violent organised groups in Italy, and they can take whole neighbourhoods hostage of their riots for a full day: this obviously drives rational and normal supporters (often with kids) out of the habit to go to the stadium.
D. Lower quality of games: many of Serie A’s matches are consistently boring as the star players are going to play in richer championships (but things are improving, especially since 2015).
E. Overwhelming football offer on TV: compared to all the difficulties raised against the legitimate supporters to go to the Stadium, the offer of Football on TV is outstanding. There is simply not incentive to go to an empty stadium, if I can watch the same game from the comfort of my sofa at home.