Germany's problem, from my experience of a Christmas market holiday, is that they have near zero concept of security.
Wander in and out, loads of places you could drive a car up, no bag checks, lots of people crammed in together with no management of numbers or avoiding a crush.
People drive cars into crowds in Germany because Germany doesn't yet appear to have got it's act together on safety, or because Germans are prioritising free unmonitored association higher.