I thought the surge was partly due to Berlusconi leaving. Attendance figures went back up right from Yonghong Li's takeover. But there's another factor, which is the overall attractiveness of Serie A as a product. During the last 3 years, the excitement and attractiveness has clearly picked up, while the years from 2011 to 2017 saw even Italians getting bored of a stagnating league. So I think you'll find a similar trend for all big teams.
We are still quite a bit behind Inter numbers though. They comfortably cleared 60K since Suning got full charge and they are touching 65K now iirc.
It is probably a factor, but there's also a strong correlation with
employment rate.
I believe success/owner ambitions are still the strongest factors.
Napoli's have plummeted after Sarri.
Roma's peaked under Rudi Garcia / back when Pallotta was popular.
As for Inter, they've been leading the attendance league for years. Moratti didn't fall out with fans the way Berlu did - he was investing until the very end and sold as soon as he had lost interest/hope.
Some try to use the current circumstances to prove Inter normally have more attendance and things are just going back to the pre-Berlusconi reality. However it's not true. Milan have usually been the ones to draw the largest crowds in northern Italy. Stadiapostcards have attendance figures as far back as 1963. Milan had higher attendances than Inter even during our rock bottom in early 80s.
http://www.stadiapostcards.com/archivio.htm
Maybe there is also some "I want to see Milan in San Siro (again) before they demolish it" attendance lately.
That's probably also playing a part.
The Sassuolo game was our 120th anniversary and beforehand we had a string of good performances against Napoli (SS), Bologna and Parma away.
I was thinking of bringing up the anniversary, however I don't think that's all there was to it.
The 110th anniversary in 2009 had even fewer attendees than the previous game. That wasn't even a bad season.