Way back when in the late 60's or early 70's, one of the original poster advertisements for Negril said to come visit the "Seven Miles of Beach in Negril" - this included the main beach, the beach at Bloody Bay as well as the 3 smaller beaches on the West End now by Canoe, across from Mi Yard and the last bit at On the Rocks.
The ad never stated an actual "7 mile single stretch of beach", but the nickname stuck, so the "Seven Mile Beach" has become the name for the main beach in Negril.
And yes, I think it was because it had a nice sound to it... (grin)