It’s a good point.
The reason is that I can figure out the contributors just by looking at the commit list. It’s much harder to keep track of the underlying contributor for an issue, and we don’t have a systematic mechanism for doing so. In the meantime, it does provide another incentive to make even a simple PR – we do credit docs updates because they end up in the same commit history.
The current macro only works for GitHub usernames in any case; perhaps we’d need a separate section for the talk.tiddlywiki.org usernames of other contributors.