It was Noah Webster, of the dictionary fame, for the same reason as colo[u]r and cent[re/er] - to put a new identity post revolution on American English
Regional dialects at first, but then the dictionary declared it the official pronunciation and shortly after the song was produced, which would have spread the use from being regional and encouraged homogenization
It was Noah Webster, of the dictionary fame, for the same reason as colo[u]r and cent[re/er] - to put a new identity post revolution on American English
Looks like it was a combo of both, reading about it further. https://www.rd.com/article/americans-british-pronounce-zee/
Regional dialects at first, but then the dictionary declared it the official pronunciation and shortly after the song was produced, which would have spread the use from being regional and encouraged homogenization