One reason you got right, it is a big metro area with many smaller cities nearby. Also, Frankfurt is conveniently located near France, Belgium, Luxembourg, Switzerland and the Netherlands (within driving distance). In addition, just being in the middle of Western Europe, it is a hub for many passengers who continue their journey beyond Germany and beyond Western Europe, for that matter).

Actually, I heard that Berlin airports are expanding rapidly to share the load. Berlin, despite being the capital and the most populated city, has been a divided and disputed city for a long time, and was also not very accessible (as it was in the middle of East Germany, which required visas for and imposed many restrictions on Western travelers). That situation gave rise to Frankfurt and its airport. But it is beginning to change.