While FRA has been a hub for LH, the proximity to and now the aquirement of Rhein Main Airbase land makes it a perfect transportation hub. And the Frankfurt metro area's population is not as small as it first appears. The only other airports within a 2 hours commute is Hahn (little more than 1 hour) and Bonn (around 1 hour).

I would also be willing to bet frequent flyer miles that the US Government also had a big say on FRA becoming a major airport with all the military bases within that 2 hour commute circle.