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To make it simple everything goes back to golden and silver age and those characters were not made to reflect the real world. Those characters represent concepts and characters most fans want to see and back catalog books and products for company so putting out popular Superman book makes DC more money than Sideways book that is as popular. Companies have two choices mostly for character they want to stick
1. Create Legacy minority(or new character) and have them replace the original character for awhile which upsets fans
2. Make the original character a minority when the medium provides a chance for it ala New 52 or movies like Aquaman which also upset fans
Creating legacy character seems like good idea until you remember that everything goes back to gold and silver age.Superhero Comics are super nostalgia based so new or even legacy concepts are hard to stick. Why not create Kryptonian of color but Calvin Ellis and Val Zod exist are not around. Because Jessica Cruz and Simon Baz are already not around, It isn't just minority characters Wally West isn't around again. Which bring us to my point
If Justice League was created today it wouldn't look like this, If X-men or Avengers were created today it wouldn't look like its original version and despite everyone of those groups looking more diverse than before when books go back to their core (which they have to keep fresh) the teams all look at certain way. If you want real lasting impact you have change one of golden or silver age characters. If you are putting characters in modern setting you have chance to fix one of comics biggest mistakes and make characters reflect the real world. An Alien Immigrant from another planet doesn't have to be white nor does kid from Brooklyn New York. Aquaman can be Polynesian make a nearly billion dollars and prove his skin color doesn't matter but a good story does.