What Marotta made clear was that Cassano is not a part of Juve's plans. Google "Marotta Cassano No" and you will find many articles about it from December 13th.
I don't think Marotta ever said that Cassano being a troublemaker is what makes him undesirable by Juve, but he clearly said that we're not interested in Cassano.
Now why we weren't interested, I don't know. Some here think that it was in order to mentain the good relations with Garrone, but I don't see how buying Cassano would have troubled that relationship, since Galliani and Garrone both said how happy they were that the situation was resolved in everyone's favor.