Because virtually every informed and experienced teacher was aware of the simple reality that the cane worked with a great many boys and hardly ever did them any harm if used appropriately. They also knew that it worked far less effectively with girls, and was more likely to do harm. Nowadays, we do actually have published research that backs this up, but with this as in many other cases, experienced teachers at the chalk face didn’t need research to tell them what commonsense already had.
Given that depression and suicide rates are greater amongst men and that boys were more likely to have received CP, commonsense would show exactly the opposite of what you claim the research says. Why do you think believing CP causes psychological problems in girls was “commonsense”?