Because there is not text book on debugging, period.
In fact, it is very hard to create a teaching situation where get the incentive to use a debuigger. Typical assignments are too simple to really require a debugger. Greg Wilson raised that topic at last year's SUITE workshop, and consensus was it is very hard to get students to use a debugger. You can tell them about debugging, but creating situation where they will actually feel the pain of resorting to the debugger is hard.
Maybe a lecture on game cracking could motive students to step through the game with a debugger? At least, that was why I told myself how to use a debugger as a 12-year old :)
