The Wikipedia article states that the Great Vowel Shift ended in 1700:
Why is it, then, that Lord Byron (to pick a name at random) rhymes "I" and "Italy," "good" and "flood," "put" and "but," "move" and "love"?
Vowels (and consonants) are constantly evolving. Writing tends to freeze a time period's speech patterns, like a fossil of a life form, but the language continues to evolve. There was a sound change in Spanish-Portuguese from terra -> tierra, fazenda -> hacienda etc. Changes are constantly happening. Why a particular artist in a particular time period made the stylistic choices he or she did is something I don't know.