Hmm, I think it might be one of 2 things.
1.) Subject driven, i.e. who has/had the more interesting designs.
2.) Local audience in the place of manufacturer.
3.) Historical factors(i.e. allies, etc.)
Japanese seem to love military modelling, and they seem to like German stuff, ergo japanese companies will produce more german stuff.
Another factor might be access to the actual item to base models on too.