Every single Ethereum miner has to process every single smart contract transaction and they all have to do Proof of Work, which takes a huge amount of computation power. With oracles, you don't need the miners to do the heavy computation - instead, a small set of elected oracle nodes do all the work, once they agree on the result, they update the main chain. With Steem, you get the additional benefit that miners (witnesses) don't have to do POW - they they have already proved their trust by the fact that they were elected by the network. So even Steem on-chain transactions are fast and cheap.