Northern German here: The reason is simple, the only people who wear helmets are the ones who know they are at risk anyway. Correlation != Causation.

EDIT: Yup, article confirms it.

In my city, the amount of trips taken by bike is over 26%, with another 12% public transport, and most younger people use bikes several times a day – but most don’t wear helmets, and most never had an issue with it, yet.