30–40 years ago, the term in British English was usually "fixed wheel" — for the same reason that you have a "freewheel" rather than a "free gear".

"Fixed gear" seems (to me) to be a modern expression, no better or worse, but a new way of saying the same thing.

"Fixie" was initially used as part of the trend for hipster urban types and at that time seemed to refer mainly to fashionably minimalist bikes with over-styled handlebars.

However, in the normal course of events, language develops, words lose their meanings, or gain new meanings, and new words take their place. Awful and awesome used to mean the same thing as each other; expect and anticipate used to mean different things. I think the overlap in meaning between "fixie" and "fixed" (either gear or wheel, or just "fixed") is now bigger than the differences.