Found an excellent blog post asking quick sellout vendors - those who were out of stock the first day, and some during the VIG period - "why didn't you bring more copies of your games?"
The answers are fairly obvious and even mundane, but it's all still very good insight. It often boils down to shipping costs, but also involves other things. One vendor came out and said, "We had limited copies at Gen Con, because that was all we could have available". Another detailed sales of previous games at prior Gen Cons along with his sales figures from Origins and simply made the best possible guess. Other things factored in - for example with that last vendor, storage was an issue - but the point is that there are reasons companies who'd otherwise love to sell as many games as they possibly can at Gen Con don't bring enough to cover their sales.
Anyway, this is worth reading. Like I said, very good insight.