Given the amount of discussion there has been about the reduction in Facebook's organic reach, the reason is surprisingly straight-forward.
People are reading more or less the same amount but much more is being created and shared. So the proportion being read has fallen.
It should be no surprise really.
There’s on average around 1,500 posts eligible to be shown in each person’s News Feed each day. Two, people only spend a limited time looking at News Feed per day. Facebook only shows around 100 posts per day. Zuckerberg says “No person will look at everything.”