The main reason is that the value depends on the steemvalue in general. A cryptocurrency is unstable and changes in worth rapidly. The same is happening to everyone.
Also another thing is that steemit distributes the income you are making with posts and comments to other accounts.
Why? Well, everyone has to be paid, it is like communism in the digital world.
Don't worry about it too much, a comment of mine made about $22 worth of upvotes and it decreased to $14 on one day and is increasing and decreasing by the minute. This is just the system. Nothing we can do really.