It was an incomplete technique that several people still learned. So that excuse doesn't hold water. Sakura was the one that wanted to catch up with Naruto, remember? So yeah, where didn't she train with Kakashi to learn it?Why didn't Sasuke learn it, when his Chakra control is on the sage's level? This is such a lame counter to an obvious manga fact. The Rasangan is an incomplete technique to begin with.