Why use the term straight instead of heterosexual?

Let me answer a different question.

As I wrote on A cis lament for the Greek language and How to say transgender in Greek, the Greek language has a Greek term for transgender, diemphylikos. Trans Greeks were involved in coining it.

The Greek peak body of LGB (with only token T) uses diemphylikos.

Greek trans groups, including the very people who came up with diemphylikos, refuse to use it, and use transdzender and trans instead.

Why? Because they did not want a self-designation that sounded like a medical diagnosis.

And while my cis Greek linguistician heart bleeds to hear it, I understand that.

That’s also why gays don’t call themselves homosexuals.

And as frustrated as they have had reason to be with heterosexuals, that’s also why they don’t call heterosexuals heterosexuals, or for that matter why heterosexuals don’t call themselves heterosexuals. It’s not a colloquial term. It is a scholarly term.

Oh, and as enough answers have already said: words change meaning, and more importantly, words change connotations. People really don’t think of straight as either defensively positive, or derogatorily negative. It’s just the colloquial term for heterosexual now; the social circumstances around it have changed, and so has the understanding of it. If the connotations of conventionality and rectitude were paramount, the expression straight but not narrow would be unintelligible.