Michael's top priority was to buy time to find out who the traitor was in his family. He could leave nothing to chance. That's why I believe he needed to look Roth in the eye, convince him, face to face, that their deal was still good, that "Frank Pentangeli was a dead man," that he would go on to Havana and hand over the $2 million. He couldn't build a convincing case any other way except through a personal meeting.