Because in nuclear fission, two particles that were formerly so close together that the strong force made them a single system, become separated by quantum mechanical tunneling through a barrier potential due to the strong force. This force is over 100 times stronger than the electrostatic or 'Coulombic' repulsion between them, so the Coulombic barrier potential is insignificant. In fusion, however, the dominant force acting between the two particles is electrostatic repulsion due to the positive charges they carry. This Coulombic force dominates the physics until the particles can get close enough for the strong nuclear force to operate at scales of about 10^-14 centimeters. The two particles, therefore, have a Coulombic barrier potential to surmount.
Essentially, fission and fusion are not symmetric processes because the strong force and the Coulombic forces have very different ranges.