It's origin is in boxing,:

An unexpected blow, as in They felt that suddenly raising the interest rate was a sucker punch to the administration . This expression comes from boxing, where it is used for a punch delivered unexpectedly; boxing great Jack Dempsey wrote, "The right lead [for a right-handed boxer] is called a sucker punch." [Slang; mid-1900s]

Here states the exact date:

An unexpected punch that catches a person completely off guard. The term sucker punch dates back to 1947 in the sport of boxing.

Why? Being coined by Jack Dempsey who just called it a sucker punch for his reasons, I think it'd be hard to find out. Try asking Jack.

Its origin is in boxing,:

An unexpected blow, as in They felt that suddenly raising the interest rate was a sucker punch to the administration . This expression comes from boxing, where it is used for a punch delivered unexpectedly; boxing great Jack Dempsey wrote, "The right lead [for a right-handed boxer] is called a sucker punch." [Slang; mid-1900s]

Here states the exact date:

An unexpected punch that catches a person completely off guard. The term sucker punch dates back to 1947 in the sport of boxing.

Why? Being coined by Jack Dempsey who just called it a sucker punch for his reasons, I think it'd be hard to find out. Try asking Jack.