I was always told because it was sharp enough to cut your throat. (Gullet) The size some of them were I think it would have been an animals as opposed to a human.
The actual origin is obscure, according to The Oxford English Dictionary, the 'bible' in such matters. It suggests the definition offered in the early 19th century - that it was originally a butcher's implement and related to 'gullet', as Oldwoman says - is possible. Certainly, the word has been used in that sense - especially in Scotland and the north of England - since the 1500s.