Because the gas might come out

is it right?? I'm sorry if I'm wrong
Because of safety? Depends on the content too. Some gas tanks are safe to be put in the supine position AS LONG AS the valve end is tightly closed. But most people would advise against doing so because it could explode. For example, acetylene which is highly flammable, when stored in a cylindrical gas tank, CANNOT be put in the supine position. Even an infrared wave from a remote can trigger an explosion from it.