Quoting KINECTaku's interview with Rare Studios:
"KINECTaku: How did you go about choosing which sports to include in the package?
NB: We did lots of prototypes, over 20 if my memory is correct, some of sports, some really crazy things. We paired up designers and programmers and said to them you go off and make something for Kinect in a week. What we found was that some of the pairs had done sports and a few of these prototypes had everyone wanting to play – the first one I remember was goal kicks, that was a fantastic moment. We realised then how compatible Kinect would be with a sports game and so we started prototyping again, specifically with sports in mind. A number of these second wave of prototypes were really successful in user research and around Rare and it’s those that you now see fully formed and polished in Kinect Sports."