Yeah, why is it always Superman who is evil? Why not Batman? That makes way more sense to me
Because Batman is just a guy with combat skills and high-tech gizmos. Evil Batman is just Deathstroke. Evil Superman is a nightmare, because he has such overwhelming power. He's the greatest hero in the world, but it would take so little for him to be the most terrifying villain in the world.
Although in the case of some creators, I think it's more a case of just being cynical and wanting to deconstruct the idea of Superman's heroism. Siegel & Shuster created Superman as a critique of the Nietzschean view of the ubermensch as "beyond good and evil" and not bound by conventional human morals; they wanted to show that superior power could be paired with superior morality rather than amorality. But there are creators who are more sympathetic to the Nietzschean (or Randian) view of things.