Because it simply won't work if you're sitting down. At least not right away. The motion sensing video game controller from Microsoft is getting ready for a debut next month and in all the coverage you see of the thing people are standing up. They're playing tennis, they're doing dancing games, but they're always standing up. They're even racing but they're still standing up (you know, like the great standing NASCAR drivers). That's because that's how the sensor system was designed and built. But wait a minute, don't people like to sit down while they play video games? And aren't some people incapable of standing up? Yeah, Microsoft seems to have realized that but a bit late. Now we're told that sitting down ability will not be available at launch but will be available in games that come later.
It's kind of like when they announced Windows Phone 7 would have cut and paste capability but not until next year. They realize people want a completely reasonable and simple feature but they realize it too late to actually put it in the thing out of the gate.