Literacy, pens, paper, the printing press.
A written culture has different restrictions than an oral culture dependant on ease of repetition from memory.
According to the University of Wisconsin Digital Collections Center:
Beowulf is the oldest narrative poem in the English language, embodying historical traditions that go back to actual events and personages in fifth- and sixth-century Scandinavia. During the long preliterate centuries when these traditions were transmitted in the form of oral poetry, they were combined with with a number of legendary and folktale elements (among these are Grendel and his mother, the dragon, and probably the hero Beowulf himself). The written text of the poem, as we have it today, took shape in England during the middle or late Anglo-Saxon period and survives in a single manuscript from around the year 1000.
An oral tradition requires stories to be easily memorised and stand repetition many, many times, and passed on to the next storyteller. A strong metre and fixed structure with helps, along with alliteration (also found in Beowulf) and isn't unique to English. For example, the Finnish national epic, the Kalevala, is based on sung oral tradition and has a fixed pattern of stresses, with much alliteration and parallelism (repeating the previous line with different words but meaning the same thing). In fact, the hero, "steady old Väinämöinen" (an almost always alliterative "vaka vanha Väinämöinen"), is himself a storytelling wizard who plays a zither and uses his song-words for magic.
Once people can read and write, they no longer have a need for the storyteller to recite a story from memory, they can read it themselves, or have someone read it to them from a text. Over time, this gives rise to more creative ways of expression.
For more, here's a paper (PDF) on Oral Tradition & Its Decline by Indira Bagchi.
Best Answer
English has been steadily losing many of its grammatical "complexities" (or beauty, depending on how much one enjoys grammar).
Thou and thee did not stress respect, to my knowledge. Whoever informed you as such probably felt that way due to associations between those particular pronouns and the King James Bible, which is probably where those pronouns are most associated with today.
Thou was the second-person nominative-cased pronoun. Simply put, it was the second-person form of "he" (subject). Its roots go very far back, but in Old English it was rendered þū.
Thee, on the other hand, was the second-person accusative-cased pronoun (analogous to our third-person "him"). In OE this was þē or þēc. Note that 'þ' is pronounced as a voiceless 'th', like the 'th' in thick.
You also has a similar storied past. It was simply the plural dative-cased second-person pronoun ēow (Old English had different pronouns for singular second person, when you're speaking to one person, versus plural second person, which we render as "you guys"). In modern English we form the dative case of nouns by putting it into a prepositional phrase like 'to you'. This was a single-word form of that.
English has lost many of its grammatical rules regarding case agreement. In concert with that, we've also lost having two sets of second person pronouns. For reasons mysterious, the language evolved this way. According to Google, in the 14th century 'you' began replacing 'ye', 'thee', and 'thou', and by the 17th century 'you' was the primary second-person pronoun for both accusative and nominative cases. Remember we still have the genitive your, which was also from the second-person plural genitive pronoun ēower.
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