Kinect never turned out to be the killer app for the Xbox 360 or One, but it has maintained a fair amount of interest in less entertainment-focused areas, including military and scientific applications. Still, Microsoft isn't super-keen to prop up two separate SKUs -- console and PC -- and has decided to nix the desk-bound version in favour of an adaptor.
You might be asking why Microsoft didn't just do this in the first place. Well, your guess is as good as mine. In a blog post on Microsoft's Developer Network (MSDN), the company announced its plans to "simplify and create consistency for developers" by doing away with the Windows v2 model.
Instead, developers can pick up a console Kinect model and hook it up via another piece of kit. According to Microsoft, high demand is the main reason for the change:
Over the past several months, we have seen unprecedented demand from the developer community for Kinect sensors and have experienced difficulty keeping up with requests in some markets. At the same time, we have seen the developer community respond positively to being able to use the Kinect for Xbox One sensor for Kinect for Windows app development, and we are happy to report that Kinect for Xbox One sensors and Kinect Adapter for Windows units are now readily available in most markets.
It goes on to mention that current Windows v2 sensors will continue to be supported, so there's no need for developers to switch for now.