Indirect descendant uses just whitespace because it’s the most common and most useful relation. Making it two characters would have annoyed everyone using the language every time they used it.
It would have nudged people in the direction of using direct descendants much more of the time, which is a Good Thing, as they are far easier to optimise, and are much less likely to match things by accident.
That would have made applying broad strokes design choices much harder, and is generally less useful. CSS is general-first, specific second. Making people use direct descendants more often would make stylesheets more verbose and complicated. It would have stymied the power and expressivity of the language.
