Although my expertise is not Aeronautics, I will attempt to answer using just common engineering sense.

A helicopter uses primarily vertical propulsion to gain elevation using the propeller placed above the helicopter over a large area to cover more ground. The long blades of the propeller allow the helicopter to propel the weight upwards at a relatively lower pressure differential. A helicopter moves at a much slower speed compared to an airplane. It is also limited to relatively lower altitude.

An aeroplane uses lateral propulsion. The two relatively smaller turbines are located on each side of the plane. They move air at a much higher velocity at a much higher RPM. The air plane uses the momentum and the nose angle of the plane to aid its lift off. It also uses its long wings and its aerodynamic shape of the fuselage to have the least amount of resistance from air and maximize its glide to stay in air. The airplane can move at much higher speeds and achieve a much higher altitude using its lateral propulsion. The airplane is unable to achieve a vertical lift off like a helicopter.

Fixed nozzle and moving nozzle