I think that if exchanges will use these two instruments at the same time, they will earn less. I just do not see any other reasons.
I don't think they would earn less in fact I think they would earn more unless they are also trading in their own markets. Just say I went to sleep and set a stop limit order and the price goes up then they made 0 commission but if I could also set a sell order at a mark the market hit then I would sell and they make commission. With tight parameters like most people would set if going afk they are almost guaranteed one extra commission from members while they sleep or go afk.
Thanks for all the replies people I will have a look at those other two exchanges!