Because the Left stick adjusts the power of the motors (up increases it, down decreases them). E.g. it adjusts throttle on the motors.
The right stick controls the pitch of the aircraft. As the aircraft pitches in any direction, let's say forward when you push the right stick up, it causes the propellers to pull it in that direction, in this case forward. That change in pitch doesn't actually adjust the power/throttle of the motors, however*
* Pedantic alert - it may result in them being adjusted, but not directly. When you direct the aircraft to pitch forward, the aircraft may also adjust the power of the motors to maintain altitude, speed based on variable winds, etc. But this is a secondary effect resulting from that change in pitch.