down voting without a comment is not helpful to both the user and the community
That might be a little true. The down-vote means it was not useful or a low-quality post. But why the voter thought so isn't said.
The reason why you can down-vote without commenting is that voting is anonymous. Do you think it is constructive to up-vote without a comment? Well, it says the voter found the post useful, but not why. Despite that I have never heard people complaining about this...
Usually you will find out the reason quite soon, since the question gets closed or answers within seconds by 5 people. That is a clear indication you should have done a better job researching.
Making a comment mandatory won't help since people would just write because. Also, it takes away an important part of voting: anonymity.