When one brings up the problem of evil (probably the worst problem there is, especially when it’s your problem) Christians trot out free will as why suffering is necessary. However they are not hesitant to use prayer to try to counter people’s free will (to get people to act justly or in their favor). From what I gather it’s harder for God to change people than non human actors and factors. As humans have mastered more and more of the world, the problem of evil is more and more on us. For example we weren’t culpable for stopping the 1349 black plague because we had no technology to do so but we are more so for the 2014 Ebola epidemic in west Africa (an Ebola vaccine research idled for ten years because a disease that only affected the most remote parts of Africa wasn’t a priority). As the culpability for the problem of evil falls ever more on us we need to redirect more and more of our prayers towards changing people. And these prayers are less likely to work hence the decline in the efficacy of prayer.