The focus on selling indulgences became so important because when the Renaissance period came, the church was losing its power, so people were able to freely buy things they wanted, even if they didn't needed them, and to not fear about being charged, beaten up, or even end up dead just because the church didn't thought its appropriate. Also, a very big factor was the fact that the people in this period, in general, had more money than the people in the Middle Ages, so the increased financial power enabled them to buy things that they found interesting but were not necessities.
