Let's imagine that we imposed a maximum rate as you suggest. What do you suppose would happen to the people who are too poor or unreliable or otherwise have a black mark against their records which indicate that it is unwise to lend them money at a low rate?

Such people would now be unable to obtain any legal loans at all, as it would not be a sound decision to give cheap credit to people who may not be able to repay them. An implementation of a cap on interest in fact harms the most vulnerable people and will only lead to the creation of shadow banking systems.

As I have repeatedly said, there is no reason why you should have usury laws at all. It seems to me that the only possible justification for such laws is to assume that borrowers are too foolish to understand the implications of their borrowing to begin with.