The reason for that partly is due to the fact the prosecution has no pertinent evidence or witnesses indicating Zimmerman committed a crime. It's become a case of tiny details, words and phrases, many of which have little to do with the shooting event.
BINGO! Usually the roles are reversed, and it’s usually the defense bogging down and trying to confuse and distract with a million and one details that are less than relevant from the real evidence. While typically the prosecution tries to keep the jury focused on the concrete facts and the law.