I received a question from someone and chose to answer it in a post for more people to benefit from the answer. 
"... I see in this month, the value of votes that have been given decreased very significantly, about what causes the friend? Many friends ask me this question and I can not answer it myself. thank you"
There isn't only one reason, but several. Some of these reasons can recur at anytime, and some are a result of HardFork changes to the Steem blockchain code.
There are 3 main reasons I can think of:
- The falling price of STEEM.
- HardFork 19 changes.
2a. HardFork 19 gave more power to SP holders which motivated self-voting.
2b. HardFork 19 changed how the vote power works. - Pay-to-play.
1. The falling price of STEEM
The falling price of STEEM makes your posts worth less USD/SBD. It's that simple. The payouts are always based on the value of the STEEM token.
When STEEM rises, your post value rises.
When STEEM falls, your post value falls.
This can happen because:
a) Bitcoin price falls which means the other tokens lose USD value as well.
b) More people are selling STEEM than buying which makes the price of STEEM fall.
Your post payout will fall, and your vote with have less value to allocate.
2. HardFork 19 changes.
2a. HF19 gave more power to SP holders which motivated self-voting
HF17 gave more power to SP holders. This means they could upvote themselves and get more money. And that's what they did.
Instead of using their power to vote for others like they did before, more and more users started to vote for their own comments.
Some people made new accounts to manipulate the reward pool and siphon rewards to their comments and posts, cross-account upvoting their comments and posts to increase their rewards/money in those various accounts. Collusion between people is always a factor on Steemit, but his new HF change made people collude more to get more money by upvoting themselves and each other.
Rather than use their vote power to reward others, they are concerned with rewarding themselves more.
With more people voting for themselves, your post payout will fall, and your vote with have less value to allocate. Even for those who vote for themselves, their vote value will go down.
Also note, that if more people join Steemit, and more people are voting, then more people influence how the reward pool gets allocated, and that also means your vote value will drop. This isn't the main reason for vote values falling though in the past month, but it may slightly contribute to that drop. The main reasons are the 3-4 I'm talking about.
2b. HardFork 19 changed how the vote power works
HardFork 19 changed how the vote power works, giving people less 100% votes that they were used to, which made them vote for less people.
The changes made the former 100% vote as powerful as the new 25% vote, making the new 100% vote a 400% vote before the hardfork. If someone changes their votes to 25%, it's the same as 100% before, and they can keep voting as they did.
All users with less than 500SP don't have the vote power bar to change their votes to 25%. So they don't keep voting as they did. Even users over 500SP with the bar chose to stop voting for as many people. Why? Maybe they wanted to upvote themselves more, which is reason #3.
When more votes are made on different posts, the reward payout on your own posts will go down.
More people are voting with 100% (400% in the former system) on their own comments, and this means more rewards are going to those comments, and less to the other comments and posts. All of the self-voting means everyone else gets less on their posts and comments voted on by other people, and their own votes on posts and comments goes down.
3. Pay-to-play
Many people are motivated to get money for their posts any ways they can, even paying someone to vote for them. They want to upvote themselves by paying someone to vote for them. The other person isn't voting for them because of the content, but because they are being paid to. It's a self-voting tool for the most part.
This has meshed with #3, where people are not upvoting their own comments to get more rewards, and they pay other to vote for their comments.
So instead of people voting on things they value from others, now some are paying for votes so they can pay themselves essentially. Less votes are going to other people, and more self-voting is happening through pay-to-play. It's not someone voting for themselves directly, but they are buying votes for themselves, so it's still self-voting.
All those pay-to-play votes for one's self, one's own posts/comments (self-vote), act like other self-votes, in that they apply more total votes in the system and that reduces the influence of your vote, making the value of your vote go down.
I hope these answers provided the information to understand why the value of your vote is going down.
If you have a question you would like me to do a post about, please feel free to ask :)