This falls somewhat into the category of having the necessary budget, but it's a very specific part of that, and much more besides. Iorek Byrnison is an armoured bear, a loyal and fierce companion to Lyra, and one of the biggest and best characters in the entire trilogy (as well as the small spin-off book, Once Upon a Time in the North). He's thus got to play a major part in any adaptation, and the BBC need to find a way of successfully pulling off a talking polar bear on a week-in, week-out basis, all the while making him feel like a real character with intelligence and emotion. It's one of the big obstacles they'll face, and one they absolutely need to overcome in the right way. That means that the CGI or whatever else they use to create him has to be spot-on, he can't just be used sparingly, and they need the right voice actor for him as well. No pressure, Beeb.