Italy had been invaded and reinvaded many time. As it has been said for a time sacking Rome became almost a hobby for barbarian chieftains. However, the differences between the development of the English and Italian languages has to do with respect that the root languages of those languages were given during the past two thousand years.
The roots that lead to the English language was most often seen as vulgar, "the gutter talk of the commoners.", as I mention previously. On the other extreme was Latin, the root of the Italian language. It was respected as the language of the Roman Church, it was the language of scribes a clerics, men of the cloth. It was also the language of the former Roman Empire. Even long after western half of the empire fallen, the dream that was Rome survived. Many growing kingdoms and empires were seen as the rebith of Rome, such as the Holy Roman Empire. Today's EU is that same dream tring to take form in reality once again. Latin was also the "Lingua Franca" ( pardon the pun
) for many centuries, it was the common language that was used by native speakers of other languages. When most regions of what is now Italy were invaded during the past two thousand years, the invaders often thought their own language was inferrior to that of the Romans, which is the opposite of how invaders felt about the English root language.
This is not to say that there are not variations in Italian. Just try to get the some people together from different provinces of Italy, people who only speak that regions traditional dialect, and see how poorly they can understand each other. There was no "Italian" language as we now understand the term until the time of the wars of unification, when the individual states of Italy were forged into a single nation again. That is when the language was codified, with the help men of learning.
Pangor