Further and perhaps why the route might work better than bristol is that a lot of the armed forces posted in Germany have their bases in hampshire and use this route. Not so sure Bristol has this market.
The point about the armed services is certainly true re the Bristol area.
LH operated Bristol-Frankfurt for 13 months from April 2008 until April 2009 using Eurowings Bae 146-300 aircraft. LH picked up the route after Flybe had abandoned it in 2007 following its acquisition of BAConnect. In fact, the route had operated continuously for nearly ten years by BAConnect and its predecessor BA franchisees.
The summer 2008 LH timetable called for 3 rotations each day of the week (21 weekly). Given the size of the aircraft this seemed a touch over optimistic. The winter saw a reduction to 2 x daily during the week and hardly anything at weekends.
Nevertheless, in the 13 months of operation just under 99,000 passengers were carried (CAA stats) and the route was axed in the face of the recession's grip, possibly with an eye to preventing dilution of the company's LHR and BHX routes. BRS was the newest and weakest, so was the obvious one to go in the circumstances.
As always loads do not define profitability but at the time there was much comment in PPRuNe and on other sites as to the high fares.
At another time in the economic cycle the BRS-FRA route might have survived, or it might have required smaller aircraft.
This shows the sort of challenge SOU might face if it is serious about a LH FRA operation.