Prediction markets are bets among arbitrary, dynamic sets of stakeholders. The price of a bet depends on the bets that have already been made. Both of these characteristics require state to be maintained across transactions in ways that Bitcoin does not allow.
Gnosis runs on the Ethereum blockchain directly. The Gnosis team can't change the number of shares you own, for instance. Decentralized applications are safer for users. It was founded by the team that built Fairlay, a prediction market site that uses Bitcoin, because they believe that prediction markets should run directly on blockchains.
