Really? No other reason why Rome is important? Really? Besides even if that was the only reason why it was important, wouldn't that be enough? The Ottomans are threatening THE Capital of Western Christianity!
You do realize that there is a certain nuance to why they failed right? It should be noted that Constantinople is not that important to Western Christianity and yet between 1261 and 1453 there were multiple attempts to take The City for Western Christianity, both through military means and diplomatic, because it was an effort to EXPAND Western Christianity and the same goes for Jerusalem. Again, in those instances they were trying to Expand their brand of Christianity.
This wouldn't be the case for Southern Italy. It would be a contraction of Western Christianity and any effort to retake it would be seen as similar to the Reconquista in Spain.
Hell, the Kingdom of Naples was traditionally considered a Papal fief! Spain had claim to it, France had claim to it, and Austria had interest in it! The idea that there wouldn't be wars fought to reclaim it is ridiculous!
I'm not saying the Ottomans couldn't conquer it, they could, and after the fact they might even hold it for a while, but they won't do it on the cheap. They'll expend quite a lot of blood and treasure to do so.
It wasn't easy for Spain, France or Austria and it wouldn't be easy for the Ottomans. Italy is a quagmire during this period and it's actually better to stay out.