What is the main reason for STALL in aircrafts?

Main reason for stalling of aircraft is due to flying it above the critical angle of attack.

stall accurs when aircraft do not have lift after critical angle of attack due to large value of angle of attack

Stall mainly caused due to separation of flow caused by the body in the medium. In case of aircraft the aerofoils are design to generate lift but stall may be caused by different reasons like airspeed drops to lower than rated speed, high angle of attack. this can either occur on the wings of aircraft of inside an engine using aerofoil design to drive an aircraft.

Separation of boundary layer from the air foil causing insufficient difference in pressures which cause the stalling condition, and the lift reduces.

The main reason of stall of aircraft is angle of attack. Every aircraft having limited angle of attack. when the aircraft will over come the limit stall will form.

As for as aerofoil is considered coefficient of lift  increases as angle of attack increases.At  some point of time lift decrease as angle of attack increases. Stall occurs when the wing is somewhat perpendicular (certain angle) to incoming air. The main reason  is instability of the engine of the aircraft to react to stall. Thus aircraft becomes instable. 

technical inability of the engine to respond to thrust requirements