Hello!

To answer this question, we must consider the following: that the Romance languages evolved from regional dialects of Latin, that is to say vulgar Latin (or the lingua vulgaris, the common tongue). Classical Latin maintained many of the same structures and influenced the structures of Medieval or Church Latin, which still held sway culturally and diplomatically for many hundreds of years following the fall of the Roman Empire in the West.

With the case of English, English evolved from Anglo-Saxon (Ænglisc; Old English) as its root, but following the Norman Conquest of 1066, absorbed many French and Norman words (and continental European Romance words via this) into its linguistic structure, but it retained the same Germanic form. There are some who believe that English is a creole language and underwent a process of creolisation in the stage from Old to Middle English, but many linguists and historians reject this because of the fact there is a large number of irregular verbs and similar changes to other continental Germanic languages.

Now! To answer your question - Italian (an interesting fact: Italian is actually the standard of Florence/Tuscany because of their literary heritage to the Italian nation) has very Latin roots because it evolved as a dialect of Latin itself. Following the fall of the Western Roman Empire, there was no other Roman institution apart from the Church to hold the language together, and because different parts of the Roman Empire were cut off from each other, and were also conquered by different Germanic tribes ruling over Latin (or Romanised Celts etc.) populations, the different dialects of Vulgar Latin began to evolve into specific dialects. It is around the time period of the mid-9th century (specifically with a text called the Oaths of Strasbourg) that you can see the languages begin to diverge (this document also shows an interesting look at Old German!)

Without the apparatus of a unified state to hold together a common language, it makes sense that the regional dialects of a language would diverge from the mother language, and each dialect would have their own vocabulary, even sometimes grammatical structures that are slightly different from one another. This goes for all Romance languages, from the large ones (French, Italian, Portuguese, Spanish) even down to the small ones. We did not see this on such a large scale in the era of colonialism because there still existed standardising forces, and because areas of the world that now speak those languages outside of their mother countries were settled by people from that country. Dialects and vocabulary will have changed slightly (such as Mexican Spanish adopting words from Nahuatl, for example) but the language itself retains the same basic form.

In essence, we can say that English is still called English because it evolved, at least in part, from Anglo-Saxon or Old English (what we commonly call "Old English" is actually Middle or Medieval English). Italian, in a similar way, comes from Latin, but because it diverged from the common dialect and absorbed words from conquering tribes (such as the Ostrogoths) and lacked a central state (like the Roman Empire) to enforce a unified language (as opposed to the Anglo-Saxon Kingdoms or the Kingdom of England), it had a different trajectory - like the other romance languages - and evolved into its own distinct being with its own heritage, similar to French or Spanish.

I hope that could be of help!

I wrote this answer last week which might shed some light on the question in addition to the answers here.

The "Why" is somewhat difficult to parse, but a phenomenon I do point out in that answer is that Latin continued to survive as the "Prestige" language for a long time even after it stopped being intelligible, and when literature in the vernacular (everyday speech) emerged, it was very clear it was a different language compared to Latin. Besides, it's also a given that any language that is actively spoken will change over the centuries.