The director is currently completing The Hobbit trilogy. He's so busy that he's not responding to emails sent by the production crew on Doctor Who, according to Steven Moffat!
The issue of Jackson residing in New Zealand was also brought up by the show's head writer:
I think he would also like us to go and make it in New Zealand! And I’m like “Okaaay… I’d rather we just flew you to Cardiff!
Would you like to see Peter Jackson direct an episode of Doctor Who? Which writer or Monster should he direct for? Tell us in the comments!