When people vote on things, the curators who vote first get a more than than the people who vote later. If there was no 30 minute delay, then the bots would all 'get there first', and human curators would be put at a disadvantage.
If you look at posts from lesser known authors (that do not have large bot followings), the curators from their posts usually vote later so the curation rewards are closer to 25%.
The problem with this is that posts with the biggest payouts will always have the lowest curation rewards because bots will always upvote them before the 30 min mark.
It's a game :)