I think it's because "shit" has acquired additional connotations, such as 1) An expression of dismay 2) Another word for "things" "Poop" bypasses that and can

I think it's because "shit" has acquired additional connotations, such as 1) An expression of dismay 2) Another word for "things" "Poop" bypasses that and can therefore used to refer exclusively to feces in a nonmedical context.