They, like Twitter, had good reasons for not allowing it, such as the risk of users editing posts after the fact, and the risk of abusing that privilege to scam other users, so on.
But their development did get stale some years back, and they probably know it, given that Reddit started chasing trends and implementing mostly-unwanted features some probably when they started focusing on trying to keep users on the site, and adding things like image/video uploading (which probably did terrible things to their development costs).