I assume it is simply because people thought they weren't good questions to ask on Programming Language Design and Implementation, whether that was because they felt those questions would be better elsewhere, because they thought the question needed to be modified to be fit for a Stack Exchange site (too broad, too opinionated, etc.), because they thought the question was dumb, or because they had a cavity filled earlier that day and they're feeling ornery.
People should really comment when they downvote, but as warren said, there's no such requirement. All we can do is wonder why they downvoted. If one of your questions was downvoted without any comments, then you could try to make it clearer, less broad, less opinionated, etc. Maybe leave a comment imploring other users to point out any mistakes to you. If you're sure there's no real way to improve it, then...shrug and move on, I guess.