I am guessing because most, if not all, of the people who originally worked at Marvel were White, and most of them weren't really focusing on cultural diversity- creating characters of color and appeasing fans who were people of color.
Either that or they didn't care about that at the time.
It could also be because most Americans are White, and I think it's assumed or presumed that mainstream American audiences are mostly White.
Marvel was founded at a time when people of color were looked down upon, discriminated against, and/or not taken seriously.
Same with DC.
This is probably why both publishers didn't create Black superheroes, or any superheroes of color, until decades later.
"In keeping with their origins as representing the archetypical hero stock character in 1930s American comics, superheroes are predominantly depicted as white Anglo-Saxon and non-Anglo Saxon American middle- or upper-class heterosexual young adult males and females who are typically tall, athletic, educated, physically attractive and in perfect health. Beginning in the 1960s with the civil rights movement in the United States, and increasingly with the rising concern over political correctness in the 1980s, superhero fiction centered on cultural, ethnic, national, racial and language minority groups (from the perspective of US demographics) began to be produced. This began with depiction of black superheroes in the 1960s, followed in the 1970s with a number of other ethnic superheroes. In keeping with the political mood of the time, cultural diversity and inclusivism would be an important part of superhero groups starting from the 1980s. In the 1990s, this was further augmented by the first depictions of superheroes as homosexual. In 2017, Sign Gene emerged, the first group of deaf superheroes with superpowers through the use of sign language."
"In 1966, Marvel Comics introduced the Black Panther, an African monarch who became the first non-caricatured black superhero. The first African-American superhero, the Falcon, followed in 1969, and three years later, Luke Cage, a self-styled "hero-for-hire", became the first black superhero to star in his own series. In 1989, the Monica Rambeau incarnation of Captain Marvel was the first female black superhero from a major publisher to get her own title in a special one-shot issue. In 1971, Red Wolf became the first Native American in the superheroic tradition to headline a series. In 1973, Shang-Chi became the first prominent Asian superhero to star in an American comic book (Kato had been a secondary character of the Green Hornet media franchise series since its inception in the 1930s.). Kitty Pryde, a member of the X-Men, was an openly Jewish superhero in mainstream American comic books as early as 1978.
Comic-book companies were in the early stages of cultural expansion and many of these characters played to specific stereotypes; Cage and many of his contemporaries often employed lingo similar to that of blaxploitation films, Native Americans were often associated with shamanism and wild animals, and Asian Americans were often portrayed as kung fu martial artists. Subsequent minority heroes, such as the X-Men's Storm and the Teen Titans' Cyborg avoided such conventions; they were both part of ensemble teams, which became increasingly diverse in subsequent years. The X-Men, in particular, were revived in 1975 with a line-up of characters culled from several nations, including the Kenyan Storm, German Nightcrawler, Soviet/Russian Colossus, Irish Banshee, and Japanese Sunfire. In 1993, Milestone Comics, an African-American-owned media/publishing company entered into a publishing agreement with DC Comics that allowed them to introduce a line of comics that included characters of many ethnic minorities. Milestone's initial run lasted four years, during which it introduced Static, a character adapted into the WB Network animated series Static Shock.
In addition to the creation of new minority heroes, publishers have filled the identities and roles of once-Caucasian heroes with new characters from minority backgrounds. The African-American John Stewart appeared in the 1970s as an alternate for Earth's Green Lantern Hal Jordan, and would become a regular member of the Green Lantern Corps from the 1980s onward. The creators of the 2000s-era Justice League animated series selected Stewart as the show's Green Lantern. In the Ultimate Marvel universe, Miles Morales, a Black Hispanic youth who was also bitten by a genetically-altered spider, debuted as the new Spider-Man after the apparent death of the original Spider-Man, Peter Parker. Kamala Khan, a Pakistani-American Muslim teenager who is revealed to have Inhuman lineage after her shapeshifting powers manifested, takes on the identity of Ms. Marvel in 2014 after Carol Danvers had become Captain Marvel. Her self-titled comic book series became a cultural phenomenon, with extensive media coverage by CNN, the New York Times and The Colbert Report, and embraced by anti-Islamophobia campaigners in San Francisco who plastered over anti-Muslim bus adverts with Kamala stickers. Other such successor-heroes of color include James "Rhodey" Rhodes as Iron Man and to a lesser extent Riri "Ironheart" Williams, Ryan Choi as the Atom, Jaime Reyes as Blue Beetle and Amadeus Cho as Hulk.
Certain established characters have had their ethnicity changed when adapted to another continuity or media. A notable example is Nick Fury, who is reinterpreted as African-American both in the Ultimate Marvel as well as the Marvel Cinematic Universe continuities."
Hope this helps.