I have exactly the same thoughts as @geekgirl. There is a high possibility that after 24 hours most of the posts become useless and there would be no one to check them. This also reduces the opportunity for an individual to get rewarded after 24 hours. If the author doesn't grab any votes in the first 24 hours, his efforts are wasted. It is more unlikely that anyone would be interested to vote on a post by compromising their curation reward when they can vote for another post that is less than 24 hours and get the full curation reward.
I'm also wondering why it cannot be made flat for the entire 7 days and why there should be a penalty on curation rewards a user gets for voting after 24 hours.