Probably because Berlin's status as a tech hub is a relatively recent development and before that the city was way behind other major german cities that had strong economies for decades. DNS.NET does FTTH but they are super small compared to MNET, so i hope this will get better in the next few years. Cable in the west of Berlin through Kabel Deutschland is fast and okay, just in the east with Telecolumbus it sucks pretty badly in some areas.

I have no problem with Telekom LTE though, seems to generally work fine and gives me >50Mbps in most areas, it's often faster than my 50Mbps at home.