What caused them

“They reflect us. It’s the ultimate popular culture of America.” said Georgia Higley, who oversees the comic-book collection at the library of Congress. “They really document what we’ve been interested in for most of the 20th Century and beyond. It’s also a reflection of the good and bad of our society.”

Comics have been altered and created reflecting what is going on in the world, altering the characters, villains and scenarios to match up with what people want to see in these fantasy worlds that just so happen to not be so different from our real world.

"Detective Comics, for example, debuted in 1937 with a caricatured “yellow peril” Chinese villain on the cover — drooping Fu Manchu moustache, sawtooth teeth, wildly exaggerated facial features. Inside, hero Slam Bradley
fights a cast of crudely imagined Chinese foes — characters with bright yellow skin, bearing caricatured names (Fui Onyui) and speaking stereotyped lines: “Velly solly. No see missy. You sclam!”
But, over 70 years, attitudes about race and gender in America changed, along with the demographics of comic book writers and readers.
In 2016, the same publisher that produced those caricatured Chinese villains reintroduced one of comics’ most iconic heroes, Superman, as a modern, cola-swigging Chinese teenager from Shanghai — “broad-shouldered, handsome like a movie star, tall but not in a freaky way like Yao Ming.” The new Superman was written by Gene Luen Yang, a Chinese-American from California (who also serves as the Library’s national ambassador for young people’s literature)."
https://medium.com/@librarycongress/the-american-way-how-comic-books-reflect-our-culture-47620e71f599

"In 2014, Marvel reintroduced Thor, with a twist: The god of thunder now is a woman. Likewise, other characters: Spider-Man now is Miles Morales, a black Hispanic teenager. Blue Beetle, created in 1939, now is Jaime Reyes, the son of an El Paso garage owner. Tony Stark’s successor as Iron Man will be an African-American woman who goes by Ironheart. Ms. Marvel is Kamala Khan, a Muslim-American student from New Jersey.
“That speaks to publishers realizing that people who are reading comics want to see characters and a story that reflect their experience,” said Megan Halsband, a librarian who also curates the Library’s comic book collection. “There are more Hispanic characters, other characters of color, Muslim characters, black characters.”"
https://medium.com/@librarycongress/the-american-way-how-comic-books-reflect-our-culture-47620e71f599