The reason Berlin is not a major hub is, largely, down to... HISTORY.

I guess that a lot of people here are too young to have lived through the Cold War years (and only know Berlin as the vibrant city that it is now).

Berlin has only been the capital of reunited Germany for about 20 years. Before that it was a divided city, with East Berlin being the capital of East Germany (a country that was almost inaccessible to us Westerners) while West Berlin was a Western World island surrounded by Eastern World land.

Until 1990 the only airlines that were allowed to fly to West Berlin were those of the Western Allied powers - the USA, the UK and France. Services between West Germany and West Berlin were operated almost exclusively by PanAm and BA (Air France withdrew from the internal market in 1969). Few if any Western airlines operated flights to East Berlin's airport, SXF.

By the time Germany became one country again FRA was well-established as an international hub, and there were really no incentives for airlines to move their hubs to Berlin - with all that would have entailed (relocating staff, re-routing feeder services etc etc). Post-reunification TXL has become a regional airport just like any other German regional airport, with direct services to major European destinations but very few long-haul flights.

I haven't seen many (if any) posts on here complaining about the lack of long-haul flights from places like HAJ and HAM... I think one just has to accept that, for historical reasons, Berlin is and probably always will be a regional airport (personally I doubt that the new Berlin Airport will make much difference, at least not in the short term)