I couldn't say for sure, but I'd bet that it's because they priced the Xbox One at $500 against the PS4's $400 and between the two options a lot of people would

I couldn't say for sure, but I'd bet that it's because they priced the Xbox One at $500 against the PS4's $400 and between the two options a lot of people would just buy the cheaper one (especuially Santa). Dropping the included Kinekt was a way to get the cost competitive.