Why was Joshua Logan dumped?

Logan hated the finished script. It was only good enough for also rans like Troy and Connie. He did another movie for Warners that same year, Fanny, which was nominated for an Oscar for Best Picture,

Logan was, frankly, a lousy movie director (for example, his Camelot is execrable). Delmer Daves had enormous success at Warner Brothers, especially during his second career there, in the late '50s through the mid-'60s, and was a far better choice for Parrish.