TXL was built when The Wall was still up as a "domestic" point-to-point airport. Lufthansa didn't even fly there from anywhere. Until today, no automatic baggage handling and very few of the airbridges capable of handling widebodies. No facilities airside for transiting passengers due to the quirky layout of Terminal A. Hopeless location for a hub.
SXF (which might merge with BER some time in the next few decades) more or less the same.