This was demanded by members themselves a while back. Some posters used to edit their original posts after a few replies were posted, remove stuff and information which made subsequent posters look like fools.
I received plenty of PMs requesting for the time limit, most of them demanding a 5-minute limit, and some demanding the ability to edit posts be removed completely. But, in the interest of fairness and for those members who wished to add additional information to their posts within a certain period of posting it, I extended the time to 30 minutes.
#4 in the list of Changes:
http://www.horror.com/forum/showthread.php?t=63919
Some of the guilty posters are still active and around. Sorry, but the rule stays for now.
The best solution I can offer is this - send the content you wish to add to your original post as a PM to me, and I will edit your original post and add your content to it, as requested by you.