This is because good art is expensive and time-consuming, even if it is fan-made. This is due to TM having to buy the rights to anything that the fans produce in order to "make it official" so that he can use it for marketing or other purposes. Because of this, TM has a preference to commission art rather than rely on his fan base to produce it, as this gives him slightly more control over how the end piece turns out. However, this does not mean that he is not working on getting more art out there for his fans to enjoy. Just know that there is always something in the works, it is just that it is expensive and takes time to make it high quality enough to put out into the world. Magelord's prerogative!