I simply lost my passion for the project and it became a chore I didn’t have time for. I decided it needed someone new that was excited about the opportunity to push it forward.
Be sure to read Iain's take on the purchase as well. It sounds like he'll continue with Brad's pivot to market WP App Store as a mailing list focused on discounts & deals for WordPress users. I think it will be interesting to see if Iain eventually does more with it, though.