It's possible the write a human romance to rival the pull of the fairies, but you've got to have Kate Sutton/Christopher Heron levels of chemistry, and Ash & Kaisa just don't get there. (And of course Kate Sutton does feel the pull of the Fair Folk - she realizes once they're gone that she'll miss them. Never mind they enslaved her underground and nearly killed Christopher! Such is the power of the "come away, come away.")

And of course showing the cruelty of the Fair Folk also makes a difference. Ash has heard stories about it, but unlike Kate, she doesn't really meet with any of it herself. If she did, that might also have tipped the balance.