There are countless reasons, but I'll name a few.

The problem of evil. There are a few examples, but I will focus on Draper's evidential problem of gratuitous evil.

1. Gratuitous evils exist.
2. The hypothesis of indifference, i.e., that if there are supernatural beings they are indifferent to gratuitous evils, is a better explanation for (1) than theism.
3. Therefore, evidence prefers that no god, as commonly understood by theists, exists.

Now, free will dictates we can do what we want, and some people do some pretty horrific things. I understand that. But the problem is with gratuitous evil. Take the Holocaust, for example. I understand that some bad things must happen in order for us to truly understand and appreciate what is good. But when is enough enough? What can we possibly learn from the extermination of 6 million Jews that couldn't have been learned by the extermination of a small percent of that? You don't think an omnibenevolent God could have intervened at some point and stopped it? What it tells me is that if there is a God, He either has the power to stop these things and chooses not to, or he is an omnibenevolent God, but doesn't have the power to stop it. The more likely scenario, however, is God doesn't exist.

Some may argue that God is a "hidden God" and therefore can't make any sort of overt contact with his followers, because faith is believing without knowing. You have the choice to believe, and God showing Himself to His followers takes that option away. But a hidden God isn't possible either. There are countless examples in the Bible itself that prove that God isn't averse to showing Himself. Exodus, Chapter 14, the parting of the Red Sea, for example.

Want an example from the New Testament? Matthew, Chapter 17:

After six days Jesus took with him Peter, James and John the brother of James, and led them up a high mountain by themselves. There he was transfigured before them. His face shone like the sun, and his clothes became as white as the light. Just then there appeared before them Moses and Elijah, talking with Jesus.

Peter said to Jesus, "Lord, it is good for us to be here. If you wish, I will put up three shelters--one for you, one for Moses and one for Elijah."

While he was still speaking, a bright cloud enveloped them, and a voice from the cloud said, "This is my Son, whom I love; with him I am well pleased. Listen to him!"

Clearly, God doesn't want to hide. So why is God hiding today? Simple, the Bible is a collection of stories created by MAN, and God probably doesn't exist.