why havent they called the dad of GZ
If you study trial practice you will learn that one of the jury's main complaints is that lawyers are to repetitive. They say that the same point is made over and over. It is understandable. From a trial lawyer's perspective, you want to be sure that the jury "gets" a particular point, so you beat on it over and over.
The defense may have decided that any point Zimmerman senior would make is already made. It is a total judgment call.