They trusted their leaders, as persons chosen by God, to handle the financial affairs of the church or agency honestly and responsibly. So the accountability was spotty at best. Too many were also just plain gullible. Further, it was extremely rare to find one who was ready to stand up and sound the alarm about a fraud scheme, whether from embarrassment or guild loyalty. Many more were anxious to hush the scandals up and divert their flocks' attention to happier topics.
Outside audits are a basic institutional precaution.
They are well worth the cost. Put even more plainly, auditors are paid to look for fraud, and thus they can deter much of it. The second ingredient in meaningful prevention must be self-education. But this should not be left to individuals. Con artists target groups, and deterring them is a group responsibility.