That's a good point regarding licensing that I hadn't considered. I know gaming companies are very protective of their licenses, so I wouldn't be surprised if Wizards was a little strict or on the expensive side.
Another issue is that other than the logo itself... there are really no current iconic images or graphics that represent Dungeons & Dragons (besides perhaps Drizzt.) It's all book covers and random fantasy pictures with unnamed adventurers. Thus, there's not much that a tee-shirt designer could create a D&D line with that would actually possibly sell as well as say Spider-Man, Minnie Mouse, or Cars.
There are a few places that are making D&D merchandise (and the website has a Merchandise page that features a few of them), but it is by no means as widespread as other brands are. My guess is that the desire or requests for them just isn't there.