What's there to explain?
All those people named have been acting for 10+ years and are popular because of what they've done on screen.
Will Smith was a rapper before he started acting. What do actors do that a rapper also does? Remembering lines and reciting them.
It is true that someone can become a decent, good or even great actor but it won't happen over night or in 4 years.
So why would Marvel/Disney who are known for great casting choices, take a gamble with their first female led super hero film?
Because they like the way she conducts interviews and she works hard in the gym?