A lot of the overhead from 'traditional' microfinance (the model popularized by Mohammed Yunus in the 70's-80s) comes from creating social/community-based incentives for repayment, referred to as social capital. This is considered the alternative to regular loans since in non-collateral lending the borrower loses nothing monetarily if they don't repay.
According to their site, the repayment rate for Zidisha is under 50%... thats pretty dismal in an industry where you normally consider a healthy MFI (microfinance institution) to be operating at high 90 percentiles of repayment.