This happens because no one has time to read 100% of the status changes, posts, shares and announcements of 100% of the people and pages they interact with on Facebook. Not enough time for all "followers" to read it all every day.
They have studied and calculated the amount of time the average user will stay there, scrolling through things, clicking, posting, reading, sharing, etc. So FB decides what is important and to whom. They decide this by assigning weight or priority to people who regularly interact with your posts. If your posts are not interactive, fewer people will see them. They also claim that telling people to like or share will lower the value of those shares or likes.
If you want more people to see your posts, you have two options: Pay for more reach or post things that spontaneously get comments, shares and likes. This, of course, is more difficult for posts about tractor parts than for posts about kittens.