There are a couple of factors at work that finished off the Frankfurt hub for DL.

1. Plane size. In the days of Pan Am they would fly 747's from 6 or so US cities into FRA and then feed the PAX out onto 727's.

2. German feed. Pan Am had an intra German airline that while primarily serving Berlin added to the feed at the German end.

2. Different hub structure in the US. In the heyday of Pan Am, many Pax had to go through a limited amount of US gateways to fly to Europe (NY being the biggest one). With the development of longer range 2 engined planes smaller pax numbers could be accomodated and could fly from ATL, CVG, etc AND therefore together with natural passenger growth, many of the cities that were served through the hub could then develop into their own nonstops (IST, ATH, MOS come to mind).

3. Alliances (A). DL made alliances with first Atlantic Excellence, then Skyteam to route their pax on other airlines, therefore negating the need to maintain their own smaller aircraft in Germany.

4. Alliances (B). Frankfurt is the main hub of Lufthansa one of the biggest * alliance carriers. Much like how the majors avoid eachothers hub in the US, the same applies here. A great example would be the Delhi flight that was supposed to start over a year ago. DL got the OK from India and was ready to start the service, they also got German ok's but were given incredibly bad slots that made the flight unfeasible and uneconomical. It was soon after that DL pulled the last FRA-BOM flight and effectively closed the hub.

5. Wages. German employees are among the most expensive in the world. In the old days of Pan Am, they were even allowed to hire Polish stewardesses (I am using the throwback on purpose) that cost about 20% of western FA's wages.

6. Military draw down. At the beginning of the nineties there were something like 120,000 US military personnel based in Germany. With dependants, contract personnel etc, this made a pool of up to 400,000 irregular commuters at times and was a good fallback for airlines in lean times. Today there is about a quarter of that number in Germany.

7. As stated above, in my estimation about 1/3 of Pan Am's flights had little economic feasibility out of FRA but were flown for other, primarily political reasons.

8. Fifth freedom rights etc. Just because a hub existed does not mean that they were able to sell O/D tickets from Germany on every route.

9. Another example that can be looked at is Pan Am's LHR operation which was also a hub with service up to 10(? maybe slightly off here) destinations but no fifth freedom rights. Ua bought all this up and has at times since operated AMS and BRU but still cannot sell for local traffic.