1. Few games were actually made with Vista in Mind
The simplest, most obvious answer, is that very few games actually required Vista to run, and the ones that did were often riddled with bugs because they failed to comply with some of Vista's security features.
2. Vista's DirectX 10 Didn't Add Much to the Gaming Experience
One of the biggest selling points Vista had to offer was DirectX 10. It was set to create a new future of even higher-fidelity gaming, with amazing visuals and sound. Unfortunately, many of the features rumored to be in DirectX 10 wound up in DirectX 11, which released with Windows 7. DirectX 10 didn't last long, because it didn't do a lot to improve peoples' favorite games.
Those two big points are the reasons why gamers just didn't want to switch to Windows Vista. There was nothing Vista couldn't do for gamers that XP already did, and most didn't see the point in paying for an upgrade that did nothing for them.