I don't remember what game it was. It was like 5 years ago when they were still taking PS2 games and (if I'm remembering correctly) Gamestop's pricing was more or less based on Supply and Demand. Highly sought after games that they had low stock on, they would often charge/pay higher rates.
Their business motive is that by giving people store credit for returning the item, those people will use that store credit, along with real cash to buy more items. They'll drive up sales volume, which will drive more revenue for MS and there partners.
They already do things like that... And those things are good for people who want to spend new money, to build towards future savings.
But it does nothing for people who have amassed licenses for games they no longer want.
By getting 10% back for old licenses, consumers would have the opportunity to reinvest money that they've already spent. with a large enough collection of unwanted games, they might not even need to do any additional spending to get something new.
That's great for people who have games in the collection that they no longer value.
There's no reason that credits for new purchases and buyback of old purchases can't co-exist, but the two concepts would cater to people in different situations.