Of all of the Latin languages, why is Italian so different than the Latin of the Ancient Romans?
Which modern Latin language is the closest to the Latin of the Ancient Romans?
Actually, Italian is a lot closer to Latin than you think. Knowing Latin, you can made out a good deal of an Italian text. Most of the inflexional endings of Latin are gone, and some accommodations to later pronunciation are made, but there is a considerable closeness the passage of over a millennium. I did not study Italian formally and yet could read most everthing I encountered in Rome just by knowing Latin. It is the same with Spanish and French. So, in studying Latin, you are actually learning at least four languages at the same time.
Compare this situation with English, which is far worse. Most users of Modern English have at least some difficulty understanding Early Modern English (Shakespeare). The meanings of words have changed, and the grammatical forms of words can be different. If you go back farther, to Middle English (Canberbury Tales), you are essentially translating a foreign language. If you go back farther, to Old English (Beowulf), you are truly dealing with a foreign language. Similarly, Modern Greeks cannot read ancient Greek or Biblical Greek without special study.
Spanish, French, and Italian are closest to Latin. Spanish has been influenced somewhat by the Moorish invasion, and Italian by several regional dialects in Italy.