Superman as the moral paragon is too "perfect" and unrelatable when combined with his powers. It is easier to always take the moral high ground when you are so powerful that nothing hurts you or a better situation is almost always possible. That doesn't always sit well with modern readers. He is too alien, too godlike, to remote.

The flip side is to try and add some moral ambiguity to make him more relatable, but to do so then he is not Superman anymore. I think the reaction to Superman killing Zod in Man of Steel is proof of that.