Why do Mermaids wear bras?

Answer 1

In the early days of folklore, mermaids didn't wear anything at all: they had fish scales on their bodies, or were perhaps more porpoise-like, and only their faces were those of women.

As time went along and sailors told more fantastic stories than before, mermaids became female from the waist up, and (though it's not recorded) they were considered to be topless by sheer virtue of being enough to tantalize any sailor to jump ship. In around the 1400's, there was a merging between sirens and mermaids, and the allure of mermaids became highly romanticized by the 1600's. In the 1800's, they were considered vulgar and erotic.

But in the 1940's, mermaids had to start covering their breasts because Hollywood didn't want to offend the "common decency" ideas that most people had at the time, and so beginning with RKO Productions every mermaid wore a bra made of shells, starfish, or something else which might be used to cover up their perceived indecency. It was never agreed that this was indecent; it was simply accepted that everyone thought it was (social norms).

By the 1980's, people had stopped really questioning the idea, accepting that it's what mermaids wore -- after all, who would want to call a mermaid vulgar?

It's only really been recently that people have really questioned this, and so it's almost certain that the mermaids intended for general family audiences will continue to wear these things to cover themselves with.