@hakesterman, HD DVD is not a microsoft format, they just happened to release an external HD DVD drive for the xbox 360 at one point. It was a Toshiba/NEC product. It's also not just as easy as reviving a dead format, since the costs would be higher to restart production on HD DVD lasers and disks instead of using Blu-ray which is in full production now.
@STravis It is for the ever increasing size of games. If they don't move to something like Blu-ray, games are going to have to come on multiple disks and require installation on a hard drive.. Otherwise you will end up with the same thing that happened with games like Final Fantasy XIII, lower res cinematics and more highly compressed audio as well as other compromises to not have to use so many disks.