English was not a tremendously unified language in England in centuries past. Not only did people have different words for the same thing:
"eggs" and "eggys" and "eyren" or
"church" (also spelled churche, cherche, chirche, cherch, schyrche, chireche and other ways) versus kirk (also spelled kerk and kyrk). "Kirk" is not mostly limited to Scotland but was once more wide spread.
Into that mess came "zeta" the Greek letter. "Zed grew to be the dominanyt pronunciation, but English speakers don't follow as many rules as some would like so there were variants like "zee" (dating to as early as 1677), "ezod," "izzard," and "zod".
Just as British slowly became more unified (and settled on "zed") the same thing happened in American English, but we did it away from the British influence that settled the question in most other english-speaking nations.