Linguists call it "the Vowel Shift", which happened during the reign of Elizabeth I.
It was when the English language moved its vowels.
(ducking)
Independently of the "original pronunciation" movement, which I only heard about recently, I wondered how English really sounded back in Shakespeare's time, and which variety of modern English sounded more like Shakespeare's. I figured it could even be American English, or some regional American pronunciation like on NC's Outer Banks into the 20th century, since our settlements go back to the 17th century, and maybe it's the English in England that changed more than in America.
Anyway, I figured it was impossible to ever know.
But then I heard about this idea of assuming that the old poems really did rhyme. Brilliant insight!