Obito needed the rinnegan to gain full control of the gedo mazou as well as have the six paths of pein.etc, sasuke was not doing that, Obito even said if they cant control sasuke its useless
Because Sasuke has his own intentions, he's way too dangerous to give the Rinnegan to. He had Nagato wrapped around his finger before Naruto got to him.
He wouldn't want to give Sasuke such power and I don't think he would accept it..
He only took Itachi's eyes because he was his brother..
Also Obito wouldn't risk giving it to someone who could easily betray him*_*