Smart Contracts are in NXT, Ethereum and Bitshares also and the decentrilized market place is also there. Why then BitBay ?
Smart contract functinality for coins is currently like having a Facebook timeline without friends.
You need more than just the option for a smart contract. The contract itself is pretty basic functionality compared to what it takes to make people use it. That's why coins like Blackcoin and Bitbay are offering platforms for people to engage these contracts on.