For Superman as he exists now to be the character that he is, he has to grow up in a relatively sheltered environment. He and the family that has adopted him can not be subject to persecution or victimization; he has to be able to empathize closely enough with dominant culture that he adopts their values & morality as a given, and he must never encounter anything as a child that would challenge that.
The occasional burglar / out of town gangster / dam bursting / forest fire / alien invasion not withstanding, Superman while growing up can not be either subject to or a witness to systematic persecution, much less systematic persecution based on racial or religious discrimination.
And historically, discrimination is what happened to people who fell outside the white Christian camp in North America, particularly rural North America.
Now, it’s entirely logical & plausible that by either the grace of God or miraculous good fortune, Kal-El was adopted by the Kents, who being white Christians in a predominantly white Christian culture did not get persecuted by their neighbors yet who at the same time were good enough as Americans and/or Christians to instill in Clark the importance of liberty, justice, and equality for all people of all beliefs.
When Superman finally makes his presence known to the world at large (we’ll presume that outside the immediate community, his exploits as Superboy were written off as the equivalent of UFO or Bigfoot stories), he has a fully formed set of ethical & moral values that could only come from a person who believes the system is intrinsically fair, that everybody who plays by the rules has a shot of success & happiness, and that discrimination is not the norm nor should it ever be tolerated.
That would not be his POV if he grew up as an African-American child in rural America. To this day African-Americans are discriminated against and treated unjustly on the basis of their skin color; if you don’t believe me, go find some African-Americans on FB and ask them to give you their POV re Ferguson & the shooting of Michael Brown.
An African-American Superman — raised as an infant in rural America — would not see the dominant culture as A Really Swell Thing but rather an obstacle that must be overcome. The fact that Superman as he exists today is lauded and praised by the dominant culture is proof he is no threat to them; if he was black & beating the snot out of racists, the reaction would be far, far different.
Let’s look at the
other side of the
Thanx to progress that has been made in America, today the possibility of the child of an African-American billionaire deciding to avenge his parents — killed in a non-race related crime — is as plausible as a white kid doing the same. Operating in an urban environment, Batman would encounter a variety of people, good and bad, from all races / classes / religious backgrounds.
Batman as a concept is also comfortable with the idea that the system — if not inherently corrupt — has been subjected to corruption that must be addressed through extra-legal means. He is not the cockeyed optimist that Superman is.
Billy Batson, altho white in previous incarnations, has also grown up in a lower class urban environment; his worldview is not as far from a lower class urban kid of any other ethnicity / religion. Captain Marvel (whom Billy exchanges places with; he does not transform into the big red cheese) is a supernatural being from a spiritual realm; he is literally above and beyond ethnic identity as practiced in North America. He could be any color — including green — and still fill his function as Captain Marvel, and Billy could be any color so long as he was from a lower class background and understands what it means to be the underdog.
If Clark Kent was not raised by wise / protective parents in the background of a dominant culture that he was presumed to be part of, he would not be Superman. An African-American Bruce Wayne or Billy Batson would still produce a Batman or a Captain Marvel.