I believe they changed it to prevent people from using large upvotes to get the curation for themselves. It didn't matter whether they were self-upvoting or using bots to do it. The author was able to manipulate it so that they got the curation. Now it's burned. It seems it would make more sense to just get rid of this time limit. A vote should just be a the same no matter when it's cast. But then I also don't know why they get burned when voting on a post more than 7 days old.