I didn't downvote your answer, rather I upvoted to counter that downvote because I found your answer useful and I think it should have a net positive score. The thing is that according to the current network-wide policy a user is not obliged to explain the reason of his/her vote. So the hard truth is that you have to get used to it. I can speculate many reasons behind that downvote. Why waste time in speculations!? Forget it, and keep movin'.
Now, since the net score of your answer is positive, you should take it as that nothing is wrong with your answer and it is ok for the site and useful to some people. Nowadays I am seeing that zero research effort question get deleted anyway, so it is always dangerous to answer that kind of questions.
Hope this helps.
P.S: I was trying to write this as a comment but it got lengthy so added it as an answer. I know that my answer doesn't answer your specific question that why did you get downvote. The thing is people vote differently as per their wish. E.g. I never downvote, and some users never upvote. So it would be difficult to speculate the exact reason.