Redoing old models is probably more challenging than creating whole new ones. The characters need to look way better, but at the same time remain recogniseable in order to be accepted by players. I for one would rather keep the old model of my female Troll (though she really, really at least needs something done with her face), than have her transformed into some character that does no longer resemble her. I want her to blink and look more lively in the face - but at the same time I want her to keep as much of the old expression as needed to not look totally different from the character I created.
More important perhaps are animations. New models need new animations as well. I love how my Troll runs, the way she shows the soles of her feet while running is what made me fall in love with the race in the first place. If that was to change, I would be seriously pissed. Same goes for her bow animation or the way she jumps. Movements I see her perform all the time and that are her characteristics to me. They are, at least to me, more important than shiny new models.
-- my guess is that they know that there is a thin line not to be crossed when redoing the old models, and thus don't want to hasten the matter. But maybe it's just me being unhealthily attached to my character(s). =)