My own opinion is that it is the range of musical styles, that arose from the interaction and combining of the following ingredients:-
the music played by the Greeks who were in Smyrna until 1922 (Smyrnaika)
the songs called "Amanadhes"
the music used for dancing by the Zeybeks
the music that was being played in Greece up to the 1920s, that had developed from folk music
any other influences I have forgotten or never heard of!.