Why did I downvote a post? Because I disagreed with the rewards for the post.
That's the only reason anybody downvotes anything, whatever their rationale is to come to that step.
It's also the only reason anybody upvotes anything.
And yes, that's a subjective preference. So are upvotes. That's what the STEEM blockchain is based on. The community of voters aggregates weighted upvotes and downvotes and comes to a final result in 7 days determining how much of the newly-minted inflation is to be attributed to a post.
Does it matter how much time you spent on the post? The amount you spent on buying votes? The creativity that went into the post? Nope.
The only thing that matters in the end is whether the community of voters, including myself, thinks it should get receive more inflation or less inflation than is currently scheduled for a future payout.
As for what authors can do to avoid downvotes? This is general advice, and not tailored to your particular post:
Engage more with the community of people that are interested in the same topics you are writing about. This is also known as "know your audience." Write to them. Engage them on Discord and promote your post organically.
If you are going to buy votes from bots, think about the value your post contributes to the STEEM ecosystem. If you are getting a 10 STU vote, do you really think your post (in your subjective preference) is equal or greater in value than other posts getting similar rewards? Again, nobody else cares what you spent for the vote - see above.
I'm not going to sit here and tell you, "Write better posts, the rewards will come." Maybe they will, maybe they won't. I can tell you that when you are engaged in a community and someone says, "You should do a post on that" and then you do, that post will likely do well.
I really appreciate your response in such a short time, regards.
Agradezco tu respuesta en tan corto tiempo, saludos.