That doesn't make sense. He was essentially the only one at GB cranking out written content regularly, with the exception of reviews. Kotaku might have more traffic, but after having another look at Kotaku just now (I tried...I really tried...but holy fuck, what an inexcusable mess in terms of design, sweet baby jeebus), I'm convinced his work will never be as prominently featured there as it was on the GB site. I clicked on 'News' and Patrick's stuff was there...easy, peasy, lemon squeezy.
GB has never been a site focused on written content and Kotaku is. He clearly loved writing more than he loved everything else at Giant Bomb. Him wanting to take a break from all the side-stuff and go back to full-time game journalism in an environment fully dedicated to precisely that sounds reasonable to me. That has nothing to do with traffic or exposure to his content, but with what he wants to do for a living.