The zed pronunciation is older, and it more closely resembles the Greek letter, zeta, from which the English letter is derived. And zed is closer to other languages’ spelling and pronunciation of the letter; for instance, the French say zède, German speakers say zet, and Spanish speakers say zeta. These are points in zed‘s favor.
The U.S. pronunciation probably came about by analogy with the letters bee, cee, dee, gee, and so on. It was standard in American English by the 19th century, and it’s now so deeply engrained that many Americans are unaware of the British pronunciation.
Australian and New Zealand English speakers usually say zed. Canadians say both.
