I had some conversation with Pete about this. They felt that if people would get them the week before, it has a couple problems.
1) people may get them and then not show. So all the early vouchers get taken & then not used because someone bails. If they give them out when you are there, they know they'll get used.
2) Sort of the same thing. It makes it unfair for someone to snatch them the weekend before & not give the guy who drives up to get a chance.
Their system is pretty much a first come, first served.