Stand with one arm and the side of one foot pressed against a wall. Can you raise the other leg side ways? If not, then why not?

With the side of one foot pressed against the wall, you cannot get your center of gravity over that foot, especially so if you lift your leg in a direction that is out from the wall.

If you have a very wide foot, you may be able to turn your leg so your body (chest or back) is pressed flat against the wall and you can raise your leg along the wall. I am not able to do it this way, either.