There are a lot of things. He develops Game Maker not as occupation, but as hobby. His primary job is as a professor. He has to do that every day. He also has been working on a book with Jacob Habgood. Also, donations definitely help spur him to develop Game Maker, so donating always helps him out. Finally, with the next edition of GM, he wants to have a set of consistent sprites of very high quality, and he has to work out contracts with some people to do this. Internationally, most likely. This will take time. He's adding so many new features, who knows when he'll be done (I'm most looking foward to alpha channels).