Because I completely forgot all about it. But good news, its still in development.
Despite being announced in 2011 for Xbox 360, PlayStation 3 and PC, Ubisoft senior vice president of sales and marketing Tony Key has confirmed to IGN that Rainbow 6 will now be headed to next-gen platforms.
E3 2013: Rainbow 6: Patriots is Next-Gen - IGN
In case there was any doubt, Ubisoft has confirmed that Rainbow 6: Patriots will be released on next-gen consoles when it debuts.
Update: This later turned into what is now Rainbow Six: Siege, the popular online squad based first-person shooter.