But it was, what, the late-1980s when she first began suggesting emphasis on intersectionality?

She probably figured she had to start somewhere. Race and sex were nowhere near as widely-discussed, back then, as they are now. But they were certainly more prominent than sexual orientation, gender identity, age, disability, and religion in terms of intersectional factors. So she picked race and sex as her starting-points, assuming that other scholars would build on intersectionality as a theory with additional attributes.

Not the approach I would have taken, if I'd been a sociologist in the late-80s; but I can understand why Crenshaw took that route.