The reason is P-factor, the torque produces by the prop blades and the slipstream from the prop hitting the left side of the vertical fin/rudder. Twist a door knob to turn it, and the door knob will want to twist your hand the other way. For every action, there is a reaction. Twin engine planes with counter rotating props do not tend to turn left. Also jets do not tend to turn left. Do a google search for "P-factor" to get more detailed explanations.