I'm guessing it's the same reason free spirits, AIs, shapeshifters and Infected PCs aren't allowed. Missions adventures are written so they can cover a fairly general (read: semi-predictable) power level. These types of characters have power levels, abilities, or inherent natures that place them outside of that central 'curve'. Often, as in the case of AIs or free spirits, they're not even able to exist in some of the adventure environments (no matrix, excessive background count, etc.). Drakes bring a whole slew of inherent abilities that place them well outside the standard power 'curve'.