As Apple said, Sollozzo felt that nobody, but nobody, would take on a NYC police captain. He felt that he was safe as long as McCluskey was at his side. Even Tom and Sonny tell Michael that it is impossible to kill Sollozzo and McCluskey. It was unthinkable.

Also, Michael was a civilian. He had never been involved in the family business. In the novel, a point is made that the other families perceive Michael as a bit "light in his loafers" and incapable of acting like the other Corleones. I'm sure that Sollozzo had no fear of Michael at that point.