Charles Taze Russell had a beard ,takeover man Joseph F.Rutherford wanted to distance himself from him its founder so he had to get rid of the physical thing that Russells followers identified him with and that was his beard.
So Joseph F.Rutherford banned beards
That`s it in a nutshell
That is also why he changed the name /heading of the Watchtower magazine from " Zions Watchtower and Herald of Christs Presence"
to
the un-scriptual heading "The Watchtower Announcing Jehovahs Kingdom"