Why Medievalism? (Part 1)

Something I've been wondering about lately: why has medievalism (or pseudo-medievalism) dominated modern fantasy literature for its entire history, going all the way back to J.R.R. Tolkien, and George MacDonald before him?  I've always enjoyed a medieval setting in a good fantasy novel and generally  avoided works of fantasy that break the medievalist mode.  I can vividly remember my childhood distaste at a book cover depicting a dragon in space.  There may have been lasers and a dragon-jet-pack of some sort--I'm not sure--but I distinctly remember thinking it was silly.  Dragons were for fantasy.  Lasers and space were for science fiction.

Of course that was a rather childish and limited view of the genres of science-fiction and fantasy...