Because if you do new message, it can get lost in someone's Junk Mail, and unless you have a correct topic, there is a decreasing chance it will be read. While with reply, it gives the "RE:" notice, which is an eyecatcher to people who have the basic knowledge of electronics, and it means REPLY. Everyone is always looking for that "RE:" to see if they had got a reply. While, they might just skim past the new message, and accidentally delete it.