The members have a vested interest in the church making mortgage payments.
Bonds also tend to cut down on people's willingness to be trouble makers.
You can sell Bonds when you can't qualify for a loan. Sometimes this can become a major problem if you start building before they are all sold and the market dries up. It takes time to sell bonds. They usually have a higher interest rate than a bank. There is a cost for selling them that is often higher than the origination costs on a conventional mortgage.