I would imagine its because explaining your downvote (unless there is a particularly good reason) also decreases the signal to noise ratio.

May I kindly ask for reasons of the two downvotes to my comment?

It was mildly annoying and unnecessary.

I precommited to explaining why I downvote anything, unless already explained by someone else.

A precommitment to breaking wind in public whenever the urge arises would be unwise for the same reason.