Quoted from PinFever:why sold out.
Easy.
Speculation buyers/would be scalpers
I don't know if there are that many actual scalpers. There is the fear of missing out on a limited run machine, combined with the belief that it's a low risk purchase. You can buy the game and if it turns out it's not for you, you can probably find someone who will buy your game for right around what you paid. I think there are those that don't buy into it to be a scalper, but when someone is willing to pay them double what they have invested it can be hard to resist.