Probably because he never placed his initials on it. It wasn't necessary for him to do so.
Really, Tim?
Westbrook was prominent in the chain of custody, wasn't he? He is the main link between the white jacket found in the parking lot and the grey jacket delivered to the evidence room some two hours later, isn't he?
Or does none of that matter and will the initials of anybody, even people who never handled the jacket, do for you?