Here's what he's quoted as saying in the Edge article:
"I'm not one to restrict the potential that Dark Souls has by insisting that only I can work on the titles. I want new expressions. It's true that I'm sad about not being involved in the development of Dark Souls II, because I've worked on Demon's Souls and Dark Souls' development for the past five years. I really love those two titles; however, maybe this is the time to have new inspiration, so I'm fine about that. I'm looking forward to playing Dark Souls II not as part of the development team, but with a little bit of distance."