There are two parts to this question.
Why was the answer considered opinion?
Much of the original answer wasn't merely saying there was no evidence. It was sharing your take on the claim.
"I'm not sure why anyone would think", "Seems kind of silly", "conspiracy nonsense", "doesn't have to make sense" are all ...
I urge readers to vote to pin the accepted answer at the top of the page. This feature does a lot of silent good work whilst the very few annoying instances where an OP picks a (seemingly) obviously wrong answer are very scarce, however memorable these are (as shown by the 9 negative answers cited in the Original Question).
Despite the terms voting and ...
I will partly quote Oddthinking answer because I disagree with part of their conclusion. Oddthinking wrote:
As you might expect, a few the OP accepted an answer that matched their priors, despite a better argued answer being available. [The change would help in these cases.]
For most of them, the accepted answer was, in IMO, acceptable. But the ...