Because no matter what system you use, more people will be without tickets than those who do have them. There's no perfect system at all, if you move to a ballot system you'll see posts on here saying "Why don't we use a first come first served system?".
The current system is more likely to be rewarding to those who are prepared and ready to go at 9am on ticket day (though it's not guaranteed to give a result). A ballot system wouldn't and would see more tickets go to people who aren't that bothered but they might go for a ticket just because it's there.