Mailing out badges before time is not possible for many reasons. One big reason is how many people might lose the badge between the time they get it and the time they arrive at the convention; or, if the mail even reaches the attendee in the first place without interruption.
There are many reasons beyond that, but I personally could say if I had to hold onto a badge for any length of time, say a week or two which is realistically when they'd have to be mailed by, I'd have a heck of a time remembering where I put it. Couple that with someone who has to travel a long way... and they forget it on their night stand. It's just not fun all around.