Because game manufacturers make 0 profit from a used sale, gamefly prides itself on buying games then renting them out to customers, then if the customer decides? Its so much cheaper used here i'll just buy it here, then the game manufacteror makes no money whatsoever. Microsoft would gain little to no profit. But gamefly would get more people definitely.
gamefly considers any of your top 5 a priority and prides itself on sending those top 5, Alot of the time it gets the first game but sometimes, if theres one that is more available to them they get that one out instead. Which sucks in my opinion.