One, bisq has relatively few users atm.
Two, I think that many bisq users share the vision of the founders/developers of a non-profit decentralized exchange with a decentralized management and funding model (the DAO). Thus, you see altcoins such as Monero and Siacoin with a consistently high share of trades on the platforms. Ethereum, being a centralized blockchain and used as a for-profit launch pad for sketchy "ICO's" is in a away the opposite of what bisq represents. For this reason, I think bisq early adopters simply aren't big ETH fans.
One + two = few bisq users that don't favor ETH
Eventually this should change as bisq gains popularity and the "normies" start using the platform for trading.