No one is ignoring anything.
First off, stop using the word THE. Like there is a sole investigator to this case. It's stupid. This isn't dragnet. Even the ABC article as A LEAD investigator. Meaning one investigator out of many.
The investigator in question was a homicide investigator that wasn't around to question Zimmerman at the time scene of the crime. Instead another investigator did it. He's was all butt hurt because he wasn't the one to do the investigation and someone had to do it for him instead. To cya, he put in an affidavit that he doesn't trust the testimony by Zimmerman because he wasn't the one who recorded his testimony.
That kind of office politics happens all the time.