One of the side effects of being big-brained mammals is that we are always looking for reasons for everything. This is great for discovering why the sun goes down or learning what makes rain fall, but has perhaps made things a little convoluted in the past when our ancestors couldn't quite understand certain happenings of the natural world.
People need an explanation for why bad things happen and the devil makes a great fall guy. Whether he is real or invented, blaming illness, accidents, addictions or any other calamity on some dark entity makes it easier to understand. Unfortunately, when we realize too late there was a perfectly scientific explanation we've already burned a few people at the stake and some nice young ladies are at the bottoms of ponds.
Firewood is expensive these days. Now we're smarter. People don't need to rely on the devil to explain the evils of the world, and fewer people believe. Sure it's a personal decision, but it's also driven by the world around us. Technology has killed God and Satan both.
But I find it interesting that believers say the Devil's greatest trick is to convince us he doesn't exist. Maybe they are right.