The reason Madara is not using or is unable to use Amaterasu is because the scale in which he would use said jutsu would be too grate. It would fodderize Sasuke and Itachi's ability to use Amaterasu.
Madara mainly uses his susano so that cut all the mountains near him. Actually, Madara would be unable to use susano on obito because Obito will suck his susano'o leaving Madara alone as a crybaby. Madara also can't use susano on grass because his susano will slip and Madara will fall down from about 500 - 600 ft breaking all his bones. U_U