From what I was thinking, a couple reasons that double bass is not implemented yet:
first of all, it's the first rock band - this gives them room to expand upon the game in sequels (not to mention adding keyboard!)
second, people need time to first get to expert on drums as it is.. it gives them time to master it now and let the difficulty ramp up more in the next game for more challenge.
third, obviously there isn't much if any metal, so why add a feature they won't use much.. but then again, this could be one of the main reasons (this, and the vocals) that there is no metal. Bah.
of course, I'd love to know the REAL reason that it's not included, but I obviously think one of these reasons could have heavily influenced the decision.