Where does the phrase "sitting duck" come from? It is a a person or thing with no protection against an attack or other source of danger.
In bird-hunting one distinguishes between shooting a bird on the ground or the water and in the air.
While one might shoot birds in such a state, (since they taste the same, or if there is a cull happening)* its not considered sportsmanlike and wouldn't count to your score if you were competing as to how many you could bag.
In WWII, when getting shot at became topical so to speak, it came to be used by both the British and the Americans (the R.A.F. Journal and Readers' Digest have uses within months of each other) figuratively of military targets that were easy to hit and hard to defend.
At this point "sitting bird" and "sitting pigeon" might also be used.
By the end of the 40s the figurative use had widened to such things as political opponents, business competitors and so on, that one would not literally shoot.
*If actually hunting, do note that in some jurisdictions it may be against hunting laws to shoot birds on the ground or it may only be allowed if for some species, etc.