I'm sure there are more than a few reasons the used games market is good, but there's one in particular I want to point out. The used market encourages quality.
The games you're least likely to find used are the games of highest and longest-lasting quality. If a game is fun, polished, dynamic and offers lasting value through replayability or the sheer scope of experience, then most buyers will hold onto their copies.
The games most often found used are the ones that are mediocre, short, redundant, buggy, etc.
I like the industry's trend toward DLC and exclusive content for new copies. But I hope used game sales stick around for decades to come, because that market encourages developers to aim higher than they might otherwise have to.