First, most genres are still filled with mostly crap and mediocrity. If you're mining for gold, you're not going to look where the gold is scarce, right? So the genre doesn't attract literary critics and literary scholars who would give new works any attention - they're all mining more promising veins. And they're not developing any sensitivities for the genre's content. So even if something wonderful emerges, no one of any caliber is around to notice.
Agreed. But doesn't this weaken your second point?
Second, the inherent nature of genre fiction is that they share many of the same qualities as blockbuster movies. That is, they are entertaining without necessarilly demonstrating any literary skill.... Contrast this with mainstream novels, which have nothing to sell except being good literature. These guys live and breath literature in ways that only exceptional genre novels dream of.
IMHO, the "mainstream novel" genre is chock full of crap and mediocrity as well. The also are "entertaining without necessarily demonstrating any literary skill".
As far as academic acceptance, I think three other factors come into play.
The first has to do with the above. In any kind of fiction, all kinds of people are cranking out all kinds of books. Out of those, few could be considered "gold" and even fewer make the notice of those critics and academics who matter, as far as their being accepted into canon. There being fewer writers in any given genre, fewer will be "gold", even fewer than that will make notice, and because the genre lacks clout, far fewer will be considered for acceptance.
The second has to do with time.
Beowulf or
Parsifal (strongly Fantastical itself) would never make notice if they came out today, but because they are centuries old there is automatically importance attached to them. My college literature course included George MacDonald, who could be classified as Fantasy, but the genre didn't really exist when he was writing so it was considered for other reasons.
Fahrenheit 451 has been around a few decades and my high-school age son had to read it for his English class.
The third has to do with the "Pop" factor. Academicians kowtow to their own trends like anyone else. Superior quality does not always make popular notice, sometimes it's just the next big noise, and five years later people wonder why anyone cared. (Remember the fuss over
DaVinci Code? And now who cares?)
Patience, friends. A hundred years from now, The First Chronicles may well make it into college level lit courses, and
The Hobbit may grace the pages of the Norton Anthology, but SF and Fantasy just don't have the cred among academicians yet. And trust me, when it does, our grandchildren will groan at having to read it the way we groaned to have to read
The Scarlet Letter. Do we really want that?