I think it's a financial issue, because teams in Italy are not as rich as teams at premier league or la liga, which means that the top are getting even bigger gap than the bottom. At premier league when you move from Championship to Premier league it is still not that easy to stay up, a lot of teams end up staying for a few years and then going down, but their lower level leagues are so competitive and loved that they still get millions of dollars, and when they win they get like hundreds of millions of dollars.

This is why I believe that the best thing for Italian teams is to build a young squad when they are at Serie B, that way they could get better as they go along, and if they go up, they are growing up, so they can stay better, older players may get success quickly, but won't keep it there.