The fact that a repost got so many upvotes means the post has a lot of value, but many missed it previously. Perhaps it would be better to allow reposts to collect karma as usual so they would have a chance to get to the front page, but have the repost's link and comment page redirect to the existing post's link and comment page (unless a certain amount of time has passed, then the repost can be its own post and have its own comments section, which is the system we already have).

I think being able to identify what's worthy of reposting deserves upvotes, too. If a repost truly provided little to no value, then the number of upvotes would reflect that and it would never get to the front page. But in this case, many people and myself included would have never found the post if it weren't for this repost.

Different batches of users are on HN at different times and on different days. Allowing reposts to collect karma would mean that every link's exposure is derived from the entire HN userbase's votes rather than a small subset of the users that happened to be online at the time of the post.