Why is Supergirl not as popular as Superman?

https://www.quora.com/Why-is-Supergirl-not-as-popular-as-Superman/answer/Ben-Skirvin

By comparison, Supergirl first appeared a full 21 years later at one of the lowest points in superhero popularity. The CCA and DC's in house censorship standards dictated that she could not headline her own comic. Consequently, her origins are a side note in an oddball issue (Action Comics #252) of her cousin's series. She remained nothing more than an occasional secondary character for a full decade. She went through three different secret identities in her core continuity and a fourth in an offshot of her Earth 2 persona (Karen Starr aka Power Girl). None of these identities worked consistent, memorable jobs. Her most famous love interest is an obscure, odd looking green alien who talks like a living computer (Brainiac 5 of the Legion of Superheroes). She died for a good long while and was completely replaced by some kind of sentient plastic thing that pretended to be her. As a character, she simply lacks the clear definition of her more iconic cousin with many core aspects of her background being nebulous or confused over long periods of time.

I do think institutional sexism did play a part. The CCA heavily restricted many aspects of the character. She had to be a virginal sister-like teenager. Had she been a grown woman unrelated to Clark, she would have been required to be focused on marriage as her primary goal. She could not headline her own books and could only appear as a secondary character in a male-led feature. There were restrictions on the clothes she could wear. The subjects she could talk about. The types of stories she could appear in.

1959 was a very, very regressive era for women in comics. She might have enjoyed wider popularity more comparable to her cousin had there been fewer outside restrictions on the development of her character.

Even so, Supergirl is a household name and remains among the most famous superheroines in comicdom. Most likely second only to Wonder Woman and maybe Batgirl.