Why aren't there aircraft with forward-mounted vertical stabilizers?
From the image you can see that a vertical stabilizer mounted behind the aircraft CG imparts a restoring moment to a yaw disturbance which increases as the yaw movement increases. If the fin were placed on the nose of the aircraft, ahead of the CG, the moment would amplify the yaw disturbance, which is destabilizing. 

An actively controlled rudder could be located ahead of the CG, but cost and complexity normally limit this to things like missiles.