It was at night. I don't know about Bangkok but in my area I think the police only work office hours and definitely not in the dark. Hence you see even less helmets at night.
I've said it before, sack the traffic police and hand it to Grab, Food Panda ect. They seem to be able to get their riders to wear helmets.
It's a shame for the family as they aren't responsible for him not wearing a helmet, if that was the case. They just have to deal with the aftermath.