See, English was not a "written" language, per se, for a long time. Like, it had an alphabet, and you could use it to write English words, but all the people who spoke english on the daily were either illiterate, or wrote in Latin or French (the two court languages, because England had french kings). What little that is written down in english is mostly transcripts. As a result, our written language and our spoken language were very close (and words were often spelled how they sounded).
but as we wrote in english more often, and people like Mulcaster and Cawdrey are starting to write "dictionaries" that are setting spellings more in stone, written language starts to seize up, while spoken language remains fluid.
I took a class on this! Fun anecdote making fun of Mulcaster: Noah Webster wrote in 1790 that stigmatizing the double negative in English is fucking stupid, because it is! "In Chaucer's time, the English [...] used two negatives. [...] It might have been well never to have changed the practice: as the common people still adhere to it; and the change has made a perpetual useless difference between the language of books and conversation." (Rudiments, p. 50)
The claim that 18c. grammarians were responsible for the standardization of the English language is generally way more questionable than many would assert (Mulcaster, for example, claimed only to codify existing consensus forms defined by "the use & custom of our countrie"), but Latin grammar rules like the double negative were definitely their fault. Combined with stylistic elaboration in the ME period (French & Latin influence), that was the beginning of real differentiation between the spoken and the written register in English. Of course, we have a lot more registers than just two now ;)