One problem is that each troll post causes an instant flame war.

1. Unbalance between upvoting and downvoting. If downvotes had to be explained but upvotes not so, upvotes would become much cheaper, although they have the same nominal value. A fairly bad post which now earns 4 downvotes for each upvote could easily be in the positives then, with consequent encouraging of low quality posting.
2. If several people downvote for the same reason, they would nevertheless have to spell it out. So they would either refrain from downvoting if somebody had already stated their reason (and therefore posts with one substantial error would be in unfair advantage over posts with several different slight imperfections) or we would earn lots of nearly identical comments.
3. People would learn to overcome the technical solution. For example, after dowvoting, one can write simply "disagree". Or "no". Others could, of course, downvote such comments, but it would cost them much more effort, because they had to tortuously explain what's wrong (I assume answering a "no" by another "no" is silly).
4. Sometimes a troll or a crackpot appears, and then downvoting without explanation is really the appropriate tactic.

These are the costs, you have already stated the benefits.