^But, regarding orchestration, Edgard Varese, for example, used drums and electronic effects frequently, and plenty of composers used guitars! Some undoubtedly classical composers even used electric guitars! Now, some may not consider Varese strictly classical, but it certainly fits under art music. There's an exception to every rule, sometimes to the point at which the rules don't matter (considering that now many classical composers use electronics).
I would argue, as would many others, that classical or art music (more generally) is arbitrarily distinguished from popular music as a matter of convenience. In general, what we call classical music has...well, many of the traits listed above!