His initial request was weak, so compliance was weak. It didn't feel like much of a commitment. ("My only request is that...")

There's lot of research into getting people to make and fulfill commitments.

If for example he had made everyone write out "I promise to write a review of this book within two months of receiving it and post said review online", compliance would have been much higher. If they had been required to post that promise publicly to their friends when they made it, compliance would have been higher still.