The reason being, it is difficult to know whether the downvote was for the content or just an opinionated downvote. For e.g. this particular question - Which switch goes to which bulb? was downvoted & the same number of downvotes were received by the answer (the OP himself answered the question).
I know that such questions are a bit controversial (or maybe I am wrong), but simply downvoting doesn't solve the issue! We need to debate/discuss, while we are in Beta, where the rules aren't very concrete! So please comment!
PS - Am not saying that we comment all the time. Some times, it will be obvious (spam, etc.)