"As for RH ballads, apart from about 5, of the other 40 or so they all first appeared in a much printed and reprinted Robin Hood's Garland in the mid 17th century, and I'm quite happy to accept that's where they originated."

In his excellent book 'Liberty Against the Law' Christopher Hill devotes a whole chapter to the Robin Hood ballads and discusses why they gained such popularity in the 17th and early 18th centuries.