This:
1. Ethereum might be too slow to handle heavy load (that is transactions on SONM marketplace) while other projects are also based on ETH --- think Cryptokitties
2. At the same time, Ethereum is the only smart contract platform that is time tested and proven at this point. Changing to another network is possible but they decided its risky
3. Transaction fees on Ethereum are too high to have an efficient and fluid spot market. On Sonm sidechain transaction will be either free or very cheap

nice sideeffect: future blockchain projects might be build on SONM sidechain

there will be two types on Nodes: Masternode to secure the sidechain. Gatekeeper nodes to secure the connection to ethereum network. If you check out their February monthly report you can find most of these details.

BTW, tomorrow Q&A session on youtube!
You can ask questions here, they might be answered in tomorrow's livestream: https://twitter.com/sonmdevelopment/status/988462923361214464