People talking about practical armor in Red Sonja dont seem to realize its practical for her. The world she lives in looks rather warm so wearing full armor would burn her up and sap her energy, not mention restrict her movements and agility. It would be impractical to have her in full armor given her environment and fighting style; just like when Spider-Man had that armored suit that slowed him down…it made it harder for him to fight
A metal bikini is not an armor, cause it serves no protection. It’s just an uncomfortable bikini.
If Sonja was supposed to wear something that doesn’t make her hot or restrict her movements, she’d go topless in some kind of loincloth, like Conan does.
Without going into complicated discussion of the character and the origin story, the reason the original Red Sonja (not the current incarnation) can fight in that bikini is “divine protection”* (related to “a wizard did it”) combined with near suicidal recklessness.
The reason she doesn’t wear regular clothes is… the costume was designed in the 70s, became iconic and until recently people kept choosing nostalgia over making sense.
* Doesn’t it seem kind of sexist that the female rival to Conan needed divine magic to be his equal? Yes. Yes it does.