1 Increased number of pages
As the number of pages on Facebook increases the amount of stories that can be delivered to a newsfeed increases and so each page gets less share of traffic.
2 Facebook's changing algorithm
Facebook keeps changing its algorithm every now and then. Facebook said they want to show the highest quality posts to its users this not every post is shown to the same number of people.
3 Facebook is trying to display relevant stories
Facebook's new algorithm changes have the goal of displaying posts that are relevant to people. Less relevant posts might get a lower reach.
4 No body shared the post
Shares can lead to more people seeing the post and so increase organic reach. If the post wasn't shared then a very small percentage of people will see it.
5 The post doesn't get enough engagement
According to Facebook help team a post needs to get enough engagement to stay in the newsfeed of people for a while. The post needs to get enough likes, shares and comments to remain competitive.
6 Your posts are promotional
Facebook said that promotional posts won't get much exposure as people don't like them. They said if you want people to see your promotional content then pay for it. (See Why is Facebook so negative?)
7 The number of posts are increasing
According to Facebook the newsfeed is becoming more crowded as people are liking more and more pages. As a result people give less attention each post.
8 Links usually don't get much engagement
Generally people don't like to leave Facebook. Status and pictures usually get higher engagement than external links. As a result the latter might get less organic reach. (See Study Summary: Facebook makes you unhappy and jealous)
9 People are liking more pages
According to an article by Facebook the number of pages liked by people increased 50% in 2013.