as far as i remember. the reason for AM is the doppler shift problem.
with FM the frequency is compressed by the movement of the aircraft due to doppler shift.
i look at it this way, FM is the wave varying/moving backwards and forwards wheras AM has the wave varying/going up and down.
So up and down is not affected by the forward movement of the aircraft.
In the old days, traffic police cars used to AM due to them primarily moving wheras, foot patrols would use FM as they couldn't run fast enough to effect the transmissions.
Not a very technical reply i'm afraid, but hope it helps.
Geoff