Fantasy sells more, especially in the States; but then, most writers don't sell very much whatever genre they write. AS for "The Faerie Queene": it's huge, and really quite repetitive, and a little of its cod-archaic verse goes a long way [it was cod-archaic even in Spenser's day: he deliberately wrote it in an out-of-date-by-1600 idiom, invented lots of old-sounding words etc]. But it is huge! In several senses of the word. I do think it's core to the later development of Arthurian fantasy, and therefore of modern Fantasy as such. See for instance: https://amechanicalart.blogspot.com/2018/12/arthuriana-fantastica-eschatology-and.html