Sorry - I had it all typed up then had to run into a meeting yesterday.
As far as Black Panther - his entire history, character, culture and country of Wakanda was loosely based around the idea of anti-colonialism, specifically from Europe. Wakanda represented the idea of what if one African country was able to fight/fend off colonialism from Europe and fend for themselves. Wakanda represented and still does, of what could have potentially happened had African countries been left alone. The continent is one of the richest in natural resources and Black Panther and his nation illustrated how one country used those natural resources to accelerate past many other countries across the world without interference.
Africa to this day, is still stereotyped as a continent full of savages with no education, resources, means etc. That's why the story of Wakanda is illuminated because it's throws everything people think about Africa out of the window. Moving a story like that to Central or South America doesn't hold because those countries and that continent don't have nearly same history of subjugation or stereotypes as Africa. You'd have to radically change the story and character to the point where it wouldn't be the same character anymore.
As far as Storm - she has to be African (she's actually African-American, her dad was an American photographer) because she was specifically created to be African-American - she was one of the first black female comic book superheroes. She debuted in the 70's and was something that had never existed before - a African-American female leader of one of the most popular comic book superhero teams. The importance of her being black transcends just her appearance in the comics. She's been the the poster child for what it means to be a strong, independent, confident, well-rounded, African-American female, something that is severely lacking in media then and now. To remove that from her would be a complete demolishing of the character.
The importance of her staying African-American is more than just "that's always what she's looked like" she's one of the defining characters in African-American fictional literature. For a lot of minority fictional characters, they simply represent something MORE than just being comic book characters because they are minority characters in a universe dominated by Caucasian characters.
Storm and Black Panther as African-American characters stand for something far deeper than I can put into words, Johnny Storm or Peter Parker as white characters. To me, it's that simple.
Whoa, that was a mouthful. Sorry! lol