For starters, I'm not sure how you ranked these as the "main Romance languages." By number of native speakers, this is how Wikipedia ranks them:
- Spanish, 330 million (tied with English as the world's #2 language, after #1 Mandarin)
- Portuguese, 178 million, #7 (Hindustani, Arabic and Bengali outrank it)
- French, 68 million, #16
- Italian, 62 million, #20
- Romanian, 24 million, #42
- Catalan, 7 million
- Occitan, 2 million (a dialect continuum that extends from Spain to Italy and includes Provençal)
There are other speech variants within the Romance group, with large numbers of native speakers, which linguists generally define as distinct languages, but may not be treated as such politically and culturally. Examples include Romansh, Sicilian, Galician and Sardinian. They do not qualify as dialects because they are not readily understood by outsiders, even though the native speakers almost invariably speak and understand the dominant language of the larger region because they are required to learn it in school. This is similar to Irish Gaelic, which is clearly not a dialect of English; few Britons can understand it, but there probably isn't one Irish person who isn't fluent in English.
As for how the Romance languages evolved, this happens in every language group. There was once an Indo-European tribe in Scandinavia that spoke a single language, Proto-Germanic. After some of the more adventurous members migrated to the main part of the continent and then headed out in opposite directions, their languages diverged into Old German and Gothic. Old German separated into Anglo-Saxon, Frankish, Dutch, Frisian, and German, while Old Norse separated into Danish, Swedish, Norwegian and Icelandic; Gothic began to evolve but then died out. (This is vastly oversimplified.)
The same thing happened to Latin. At the zenith of the Roman Empire, there was so much formal and informal contact between Rome and the outlying provinces, that the uniformity of the distant dialects was constantly reinforced by fresh communication from the hometown. Language follows the coin, and to a lesser extent the flag, so the influence of Imperial commerce and the Imperial government kept the citizens in the far corners of the Empire bound into a network marked by constant communication.
The same thing is happening in reverse now in many dialect continua. With electronic media propagating news, scholarship and entertainment instantaneously across national boundaries, with live newspeople, scholars and entertainers following only a bit more slowly on jet planes, the differences among American, British, Australian, South African and Indian English, which in my own childhood were great impediments to understanding, are shrinking rapidly. The same is true of Spanish in Latin America.
Education had little to do with the divergence of Latin dialects, since it barely existed in those days. Only a minute fraction of the population could read and write, so for everyone else "schooling" was rudimentary and only available to the wealthy. As the Roman government collapsed and the Legionnaires either found their way home or assimilated into the local populations, the Imperial commerce network crashed with it and there was an awful centuries-long economic depression.
Without contact with people from other regions, dialects naturally diverge due to a variety of forces.
- The substrate of a colonized region's original language, e.g. the influence of the Germanic Frankish language on French. This is why it has umlauted vowels, the gargled German R, and a preference for the present perfect (I have gone) over the preterit (I went).
- A superstrate of the language of a group who migrates in after the colonizing power is gone, e.g. the Slavic influence on Romanian. This is why it still retains the Latin declension of nouns in five cases (alone among the Romance languages), has the Slavic vowels î and ă, and borrows Slavic words like do for "to" and şi for "and."
- The advance of civilization. As life becomes more complex and entire new fields of occupation and interest arise, linguistic paradigms left over from the Stone Age become a hindrance to communication. Latin, like Proto-Germanic and all early Indo-European languages, retained much of the cumbersome Proto-Indo-European grammatical structure, which became increasingly out of step with the communication needs of its speakers. You seem to have some knowledge of Latin, so surely you and your fellow scholars have experimented with trying to carry on a 21st-century conversation in the language. It can be done (it's the official language of Vatican City) but it's a big pain! The resources of the language in which we think can't help but shape our thoughts, and an ancient language is more of a hindrance than a resource for the shaping of modern thoughts.
- Spurious changes. Slang, jargon, unconscious modifications, and just plain silliness can take a language in new directions. C before I and E was hard (pronounced K) in Classical Latin. But it softened over the centuries, and not in a single way. In Italian and Romanian it became the English CH sound, in French and Portuguese it became S, and in Spanish it became the English TH sound. Linguists diligently try to explain these shifts because it's their job, but when it all comes down, it just happened.
- These changes may seem like "simplifications" to us, looking back over 2,000 years, because the intricacies of Latin grammar seem superfluous. But the languages have become more complex in other ways that we don't notice, because they have adapted to the complexities of modern life, just as English has. Look at the new grammatical construct that arose in the 20th century, the noun-adjective compound, such as user-friendly or fuel-efficient. The Romance languages have similar new grammatical features.
- I suggest a different point of view. Why haven't languages like German, Greek and Russian undergone this kind of simplification? They are hamstrung by Stone Age grammatical paradigms. Their verb conjugations and noun declensions are daunting to foreign students, and get in the way of efficient communication. German syntax, in particular, with its Schachtelsätzen (nested clauses) is outrageous.
- As I have often noted, the Chinese, with their unbroken 4,000-year continuity of civilization, have simplified their language to the point that it is a magnificient communication tool. It has no inflections for gender, tense, case, number, no articles, prepositions or other "noise words." Every word in a Chinese sentence carries an important portion of the meaning. As a result, Chinese sentences are shorter than ours and the language can be spoken more slowly, making it easier to understand.