Simply because Sandy stole the savory snacks Sally would sell in place of the seashells she sells down by the seashore.
Cost of operating tourist shop selling sea shells, bottles of sand and assorted imported junk that idiot tourists buy is far less than a burger joint. The return on investment is much higher so Sally is really pretty smart
sally is broke and sea shells are prettier
Why would she sell them by the sea shore? can't I just find them there for free? Why pay sally anything at all?
Burgers have a cost for her to acquire them & cook them - she can sell seas shells cheap cause she can get them for free
Well, if she sold something illegal, she'd make the big bucks.
Lack of commercial skills