Alot of it is down
A) Price, especially in the south people are alot poorer and therefore cannot afford to go regually
B) Nearly every match is on TV, when I was in Milan last december there was football on the telly non-stop, 'SKY' Tv in Italy is insanely cheap
C) Most Big teams generally get big crowds in, Milan and Inter usually get around 40-50,000 in for games but the stadium still looks half empty as it is 85,000 capacity.
D) these things tend to fluctuate, during the sop called 'golden age of Serie A' in the late 80's early 90's attendences where very high, higher than the prem/div1 I think and but they have steadly dropped over the last few year, and in the same time prem attendances have grown. I actually heard the other day that overall prem attendeces are in decline now aswell.
E) Juve are a team notorious for having and empty stadium this is because 1- the stadium is terrible and the view is rubbish. 2 - They are not seen as the proper Turin team by the people of the city. Torino who in the 40's where totally dominant and thought to have been the best Italian team ever until they were killed in a tragic plane crash are still more supported in Turin itsself. Juve draw alot of thier support from all around Italy, esp in the south - they usually play thier last 'home' fixture in Bari and nearly always get a full house.