SF uses Terra, as well as Sol and Luna because astronomers do. Those are the official names dating back centuries.
In common english usage we use earth, moon, and sun, but we also speak of solar system and lunar landings, implicitly acknowledging that those are the bodies' names. Any planetary satellite can be a moon but there is only one Luna. Any star can serve as a sun, but our home is Sol.
So the real question we should be asking isn't "why terra" but rather, "why earth at all"?
Besides, Terran sounds more substantial than earthling, less geeky than earthian, and less likely to be blacklisted in the bible-belt than earthy.
