You have basically answered your own question. Hertha stumbled upon the biggest city and the biggest stadium in Germany and they have always been what is commonly called a "grey mouse" in German. They were never really successful (quite the opposite actually), they never really polarized, they never really stood for good football, they never really had iconic footballers, their club anthem is basically some depressed guy singing "we're not going home" over and over again, they could never fill their stadium and their fan culture is probably ranked just one step above the plastic clubs.Why is this anyway? Seems really odd, capital city club, historic ground. Quite an old club too.
Now they are getting some ridiculous investment (yet again) because of the privilege of being the capital city's football club and they are making a proper mess of it.