Why does "sucker" mean "unexpected" in "Sucker Punch"?

The sucker here has a much longer history than some would have you believe. Basically, sucker punch derives from one of the meanings of the word sucker (NOAD):

sucker 2 informal a gullible or easily deceived person.

Therefore, a "sucker" would be easy to fool with a surprise punch. Etymonline dates the use of the term to 1836, and it appears to derive from the expression for a young mammal before it is weaned, i.e. one that is still sucking at its mother's teat.

Note that despite what Thursagen states, Etymonline also gives different dates for the term suckerpunch:

suckerpunch (n.)
also sucker punch, 1926, from sucker + punch. Figurative use by 1929. As a verb by 1942. Related: Sucker-punched.

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.