Anyone with at least 125 reputation on the site is allowed to downvote. Unlike answers, downvoting a question doesn't cost the voter any reputation. You can try asking politely in comments, but users are not required to explain their downvotes, though it's encouraged to help in improving our posts and the site as a whole.
When you get a downvote with no explanation, take a moment to step back and look at your post, try to consider other perspectives or see any improvements you could make. But don't take it personally. Different people will always have their own ideas about what makes a good post and the voting system is designed to develop an overall consensus.
In the case of your recent question, it's hard to say why someone may have downvoted it. Airplane identification is on topic and as you said you provided a good picture and extra context as well. My impression would be that the DHC-2 was a fairly well produced aircraft, and at least in some areas is still seen often, so not all people might find the identification to be interesting and useful.