Because straight is just your sexual orientation. Cis is from the Latin for something on this side, and trans on the other eg Cisalpine and Transalpine Gaul - the first was on the Italian side of the Alps, the latter on the other side. Hence cis is what you are born as, then your egg cracks and you cross over and are now trans and then get to claim that you were magically trans all along, because stupid. And then as a man in a frock you can now claim to be a lesbian, because you still want to fuck women who aren’t into you with your girldick, because devious manipulative asshole.
I am making no judgment on the worth or otherwise of these terms, merely noting usage and meaning. Oh hang on, I think I just did. Oopsie doodles.