They cause too much overhead for teaching. A minimal OS is a great way to show how one works, what the essentials are, and even what weaknesses it has.
It's amazing, when you look at a simple operating system like TinyOS, just how simple some of the things are. (Once you start actually hacking on the OS code, you realize that they're also very finnicky and easy to mess up if you don't know what you're doing. And hard to debug.)