A user wants to accept an answer, but he can't, because of a delay of 8 minutes. Why must he wait?
See the comment at the bottom of my answer, please.
I imagine to try and mitigate the "Fastest Gun In The West" issue, where one person will very quickly answer the question, and another will take more time to giving more details. Obviously both are great for the asker, but if you have to wait a while to mark something as accepted you're giving the one who's putting effort into explaining the answer a chance. However, that's a baseless assumption, and I could be horribly wrong. :)