the general basic explanation is German culture and local patriotism. City identity is very strong with Germans due to their history. Up to WW2 the working class in Berlin was not organic and kept faith with their old cities (some neighbourhoods in Berlin still reflect it).This Union - Bayern game is pretty gash.
On a side note, does anyone know if there’s a historical reason as to why Berlin is pretty much the only major European city without a big football club powerhouse? Always wondered why that was.
Until 1994 West Berlin wasn’t even a part of Germany proper and it had less working class people so Herta never lifted off. The East Berlin clubs like Union have local support but have 40 years of under investment to push up against.
Btw until PSG was bought up it wasn’t even a medium club by European standards...