Reason 5: he performed his rescues for free. Proof he has socialist tendencies

Reason 4: he is anti-military.  He destroyed a government drone and refused to join America's military

Reason 3: he does not persue wealth. He has great powers and should be using those to further his means

Reason 2:  he is not pro America. He fights for the whole human race.

Reason 1:  he is an abortionist. The scout craft was filled with thousands upon thousands of Kryptonian fetuses. Not the genetic material, but actual fetuses. He aborted those with his heat vision when he, free of charge and for the world, blew up that scout ship.

With these five reasons, every real conservative should reject Superman.

Bonus round: he is an illegal immigrant