I think DFW has always been an important market for Lufthansa. They opened the route on 1st May 1980.
It must partly due to the historical links between Germany and Texas.
However it's interesting to note that Houston isn't a very strong LH market, despite it's in Texas either.
What about the MEX and IAH routes? When were the continuation discontinued in favor of non-stop flights?
I knew about the IAH continuation, but are you sure about the MEX?
BTW, as information (openning of the route):
IAH - 2nd June 1985
MEX - 5 May 1966
When did the 747-400 operate the route?