I beleive a Lich quest requires an Imm to create it, and they are generally much harder to complete. Even with everything right I think Daev said the odds ae about 75% success on the becoming for a lich (not sure if mummy odds are higher)
That is why most go mummy, no imm involvement needed and generally easier.
That's also why mummies really are nowhere near as powerful as liches, at least in my opinion.