That's almost a chicken and egg question Beren. My guess is that Tolkien (and Lewis too) lovers were probably raised on Fairy Tales and Greek/Norse/Celtic Myths in children's books format. This probably inspired them to read more, move on to Tolkien, and thus become more educated in the process.
Reading Tolkien in particular requires a certain level of intellectual sophistication
. And as I subscribe to the notion that Nurture can only help people reach their own Nature's potential, it's really a bit of both.GB