I don't know that there is an easy answer as to why modellers prefer the Southern parts of germany to model, but part of that maybe because Trix and Fleischmann were originally bassed in Bavaria and Marklin in Wurttemberg, so maybe it is logical to assume that their product may have been biased towards their own region???

Of course RTR also plays a huge part and it has not been until Brawa started producing the stock that modeling Wurttemberg has become a lot easier.

Cheers

John