There are other things involved. Reynolds had one of the largest payouts last year, but was about 1/2 the size of BG. There are a LOT of racers that just don't want to travel more than 4-6 hours no matter how large the payouts. It's just too big of a monetary gamble if they don't win.
As the RRA for TSO, I (actually Holly) am required to solicite payout money from vendors. Each year this becomes more and more dificult. Heck, most of the vendors won't even show up and vend at the event, let alone sponsor a class. There really just isn't that big of an incentive to vend at an event when the customer can just pick up the phone and have the part on his/her doorstep in two days. If you don't beleive me, just ask most of the vendors.