As well as the price of Steem decreasing, as others have mentioned, the reward pool is a fixed value. The more upvoting that happens then the thinner the reward pool is stretched.

You'll probably notice the post value continuing to decrease in value for the whole 7 days after it's peaked but there's always a chance that the price of Steem will rise in the meantime which should result in a higher value being displayed on your post.

Hope this is useful!