The best answer is because of the difference in physiology aspects (factors) that will affect the transported sick passenger(s). As we all know, many conditions are different if we compare between ground and air transport.
As we go to higher altitude, we will enter an unfamiliar domain that will be felt in our bodies. less pressure, less oxygen, and many other things. These variables impose potential instability on the condition of the sick passenger(s).
That is why the doctors who are specialized in Aviation Medicine have been trained to deal with these variables to have the best health preservation possible.