Six works because you don't need a figure for the base. Thus, zero to five fingers on one hand, then drop all five and raise one on the other to make six. (Plus, you get easy divisibility by seven, which beats easy divisibility by eleven.)

Edit: Binary, the logical extension of the above principle, has the problem that the ring finger and pinky have a mechanical connection, besides the obvious 132decimal issue. ;)

I don't see how eight comes in, though.