It would certainly be possible to eliminate the registration fee, and make up the difference with a slightly higher recurring service fee. (We are not yet supported by any major foundations.) There are two main reasons we have not done so:
1. As a small nonprofit with limited financial resources, one of the measures we've adopted to ensure that we always have enough cash to cover our costs as we grow is to couple service fees closely with actual expenditures. There was historically a large up-front cost to admitting new borrowers, as we used to contract with local partners to verify applicants' credit histories with local lenders. We no longer work with local verification partners, but even today there is an up-front cost in Skype and SMS charges, as we verify the information of new borrowers with local contacts. Coupling registration fee income with the cost we incur to admit new borrowers ensures we will always have enough revenue to cover our costs, regardless of the pace at which Zidisha grows.
2. Shifting a portion of the costs borrowers pay to join Zidisha to a lifetime membership fee paid up front makes strengthens the incentive to participate responsibly with Zidisha over the long term, because costs of borrowing decrease as members build up a track record of on-time repayments over time.