[makecheck]

Occasionally, a single comment can get over 100 points. The site may not want to keep track of that many links to a single post. (Not that I know why it works the way it does, but this seems like a good technical reason for it.)

[mzl]

It still needs to keep track of it for purposes of stopping double voting.

[ygd]

Sites like reddit can garner thousands of votes on a certain comment. Keeping track of these would further decrease reddit's already slow response time.