There are a few reasons. But the main reasons are that the value of steem is going down these couple of days which decreases the value of our potential payouts. The other is that the rewards pool is finite, so when a lot of whales upvote there own comments all the time it takes away what is available for the rest of us. I explained this in a couple of articles..
https://steemit.com/steemit/@yoda1917/and-the-steem-comes-tumbling-down-time-to-sit-tight
Follow me and see some of my other work @yoda1917
See you there :)