The empty weight of the Q400 is around 17k kg, the empty weight of an Atr72-600 around 13k kg. Even if you fly exactly the same flight profile with an Q400 as an Atr72, someone has to pay for flying around 4k kg of aircraft. Bigger engines, landing gears, everything. For a Q400 the extra performance has to pay for the higher operating cost somehow, otherwise it's useless deadweight.