It's a regional thing. From my understanding, they used what was abundant in their area. In LA, they had crawfish and andouille, in the Chesapeake region, they had crabs and corn, and in South Carolina's low country, they had shrimp and pork sausage. Other ingredients were added/subtracted according to what was being harvested at the time. Potatoes and corn, okra and whatever other produce happened to be handy.
As the world shrunk, various things became readily available from far afield. I'm guessing the original Gullah people of the low country didn't include a lot of Alaskan snow crab legs in their boils, but it's almost required nowadays.
Now, can anyone tell me why a low country boil is also known as "Frogmore Stew"? (I really have no idea)