Decentralised exchange isnt easy to implement on ethereum, they encounter problems, its going to be very costly to do market making with the usual bid and ask structur, if u move your order it will cost u a lot actually. Its a one tier system, Waves wants to implement such a system where a node executes only what it needs to execute making it more practical.
On Ethereum possibly because of the gas price, however on the Eth platform couldn't you just create a token like the DAO, and do it? Either way I like them both.