I know very little about the italian reformation. Italy and certainly norther Italy is very similar to the Netherlands and Flanders*. Why did it never became protestant? Avoinding that, would be the first step in making Rome fall to the protestants.
Well... cultural issues and proto-nationalism play a role. There was a level of cultural estrangement from perceived "Italianness" of the Papacy in Northern Europe upon which the Protestants could latch upon. That was obviously not the case in Italy. Catholic authorities had a far easier time at enforcement in "core" areas (also, all local rulers were happy to toe their line, except, at times, Venice, while in Germany many princes chose the Reformation). There
was Protestantism in Italy of course, and usually of a Calvinist bent, like in the Low Countries and for similar reasons, but it never got the time and opportunity to gain enough traction to stabilize (with small exceptions). The fact that both politically dominant power, France and the Habsburgs, were firmly Catholic, and the Habsburgs were seen as bulwark against the Turk by many Italians is probably part of it (and that was big issue for Venice, the least Papacy-aligned of Italian states most times).