Comments are at the bottom of the article page, although people who vote 3 or higher don't necessarily need to leave feedback.
This is an ongoing issue and there have been plenty of suggestions for how to fix this. The problem, though, is that some people just don't want to take the time to leave feedback. I've written a couple articles myself and people do tend to low vote for no apparent reason. Although sometimes people just have a different methodology for how they vote and, if they do give the reason why they low voted, you may not like it or agree with it anyway (and there's usually no changing their minds).
FYI, if somebody leaves a vote of 1 or 2, they will be forced to leave a comment, which at least gives you an idea of who voted so low. There are a whole bunch of threads in the "Site Bugs / Suggestions" message board that cover this topic (one very long one was left by me), if you want to go searching through there for more reasons why things are the way things are.
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