The honest truth is CM was fair unknown-by the general public....
Along with Hawkeye, BW, Vision, SW, WS, etc etc....
None of which really could carry a title for long on their own.
Sent from the inside of a giant slug in outer space.....
Didn't help when Kevin Feige said this about CM.
" We thought it was the right time to finally introduce Captain Marvel to the world. She’s one of the most popular characters in our comics, she’s one of the most powerful characters in the comics and will be the most powerful character in the Marvel Cinematic Universe."
" one of the most popular characters in our comics" Sorry Kevin. she was never liked, least not the most popular. And you wondered why the comic folks were skeptical.