This has actually been brought up in the working group mailing list, way back when, in this thread.
It basically comes down to, yes, intuitively a selector with a combinator looks more specific, but an algorithm, extended form the current one, with this in mind becomes much more complicated than the "simple" triplets used now, which is pretty confusing for people as it is.
Finally,
While this could have been the case, this is one of the few things in CSS2
that have been interoperably implemented for years, and therefore won't
change in CSS2.1.
"If it ain't broke, don't fix it." seemed to be the final call.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…