Comentario oficial
Because the DJM-900NXS2 is our top-level mixer and was never intended as a rekordbox performance mode device, which is why a key was never included before. The majority of installations or buyers were using this with a pair of CDJ-2000 (or NXS / NXS2) as a stand-alone system, and in many of those cases, the users were not the buyers (eg. a club install), so it was considered equipment that required a purchase to unlock performance mode, and has remained such in rekordbox 6. We have asked the product teams to consider this but haven't heard anything back yet.