why is it advantageous for the stomach to be below the diaphragm rather than above it

Answer 1

If the stomach was above the diaphragm, you would have PROBLEMS !!!
- Your lungs would press against it, increasing chance of vomiting every time you breathe. The stomach wouldn't be able to expand as much as normal.
- You would have trouble breathing. The stomach must stay below the diaphragm. When you breathe, air enters into the lungs, and the lungs can dilate, because the diaphragm lowers down. If the stomach was between lungs and diaphragm, your lungs could not expand as much as they normally do.

hope that helped