It has no downvoting because it was modeled after similar code used in BitShares. Apparently the BitShares version did allow downvoting in some early form, from what I've heard, but apparently it was removed a long time ago(I'm not sure if that was before or after it was initially deployed on BitShares). I also don't know the specifics of why that happened.

Some people have asked for individual downvoting on proposals, but it's never seemed to be a big issue to me personally, so I didn't want to devote coding/testing resources to it. If someone wants to add support for it and add suitable tests, it's fine by me, as I have no strong opinion on the feature one way or the other. Other stakeholders can weigh in on the subject if they have an interest.