It's just no developer has programmed a game to take advantage of more than two for now. That's according to Microsoft's Kudo Tsunoda talking
to VideoGamer.com. The former head of EA Chicago who now works at Microsoft talked about how the experience, for now, seems to dictate two people using Kinect
but it's certainly not a limitation of the product.
An example would be Dance Central
, where it shows a few more than two players when I was watching it at E3. It's probably going to take a good amount of design to develop a good game using more than two folks in front of the system but I don't doubt we'll see more than a few here soon as the developers get more comfortable with developing for the product.