From Paizo's CEO letter: [Empahsis mine]
Many people have asked why Paizo doesn't just create new print magazines that would be just like Dragon and Dungeon, but without the official D&D content. Well, if there were a viable way for us to transition to a new magazine format, we might have. However, you can't just create a new magazine and have it automatically appear in all of the places that carried Dragon and Dungeon. Hobby store managers and gamers might understand the value of a new magazine, but what about bookstore managers, military suppliers, major magazine distributors, or advertisers? You've basically got to start over from scratch with them, and in many cases that means you have to buy your way in. When we started Paizo five years ago, Dragon and Dungeon magazines had a combined 40 years of inertia behind them, but in today's marketplace, starting a new magazine on that scale would take more than a million dollars.
Besides the money issue, a significant reason that Dungeon and Dragon sold the levels they did after a certain amount of time was that they were named 'Dungeon' and 'Dragon'; brand-recognition at work again.
Now, I seem to remember something about their licence from WoTC rendering them unable to produce a magazine for a certain amount of time [As I understand things such agreements are generally pretty common for license holders, so that they don't use your product to train themselves to become your replacement -- in my job, I operate under a vaguely similar restriction] even if they had the resources (and this is why Pathfinder has the look and feel of an adventure supplement instead of a magazine-like feel), but I can't find corroboration so that should be filed under 'speculation'.