One of my questions from October 16, 2019, Why is the ångström not a metric unit? And why is the ångström spelled with the Swedish/Finnish letters "å" and "ö"? has got useful answers. However, my question is downvoted to -6. I cannot understand why my question has got downvotes.
I did not see your question until now, but it is easy to see why it was downvoted.
The part about why is it spelt that way is a question about language, not physics. It could even be opinion based. So that's two possible ways that part is off-topic.
Why is it not a metric unit (in fact it is) is a question only the people who make decisions on the relevant committee could answer. Again it's likely opinion based.
You could have done no real prior research as Wikipedia's page on that unit states it is part of the metric system of units and the page on the metric system also lists it. That unit is, however not part of the SI system of units, so I suspect this is the route of confusion. The SI system is not the same as the metric system, although they are related.
Some members do not have enough reputation to vote to close a question as off-topic. That does not mean they'll just ignore questions they think are off-topic and it would not be unusual to find such members downvoting instead.
Note however that members on SE can downvote any question and are not required to state a reason or explain their actions. We expect them to act in good faith and that's all there is to it. I suspect some automated checks on the site monitor for patterns of voting that could be attacks by one person on another, but I do not know that for certain. It is polite to either comment a reason for downvoting or vote up an existing comment that already explains your reason, but it is not required.
It is quite common for questions to get answers (which are themselves voted up) and yet the original question is closed or downvoted heavily. Getting an answer means the question interests people, but some people being interested does not mean it is on-topic or cannot be even more disliked by a lot of people too. This is the nature of forums.
At the end of the day, you can politely ask for people to explain their downvotes, but you cannot force the issue and it really does happen us all from time to time.