Also Frankfurt am Main was the location, where the King of the Romans (German King) was elected, after the Middle Ages the coronation, also occurred there.
To be precise: Since 1147, most elections happened in Frankfurt. In 1356 the Golden Bull fixed them there.
To be even more detailed:
Ludwig das Kind, elected 900 in Forchheim
Konrad I. elected 911 in Forchheim
Heinrich I. elected 919 in Fritzlar
Otto I. anointed as successor 930 in Mainz
Otto II. elected as co-king 961 in Worms
Otto III elected 983 in Verona
Heinrich II. elected 1002 in Mainz
Konrad II. elected 1024 in Kamba (near Mainz)
Heinrich III. elected 1028 in Aachen
Heinrich IV: elected 1053 in Trebur (near Mainz)
Heinrich V. elected 1105 in Mainz
Lothar elected 1125 in Mainz
Friedrich I. elected 1152 in Frankfurt
Heinrich VI. elected 1169 in Bamberg
Otto IV. elected 1198 in Köln/Cologne
Friedrich II. elected 1196 in Frankfurt
Heinrich (VII.) elected 1220 in Frankfurt
Konrad IV., failed election 1235 in Mainz, elected 1237 in Wien/Vienna
Wilhelm von Holland, elected 1247 in Worringen (near Cologne)
Richard and Alfonso, both elected 1257 in and near Frankfurt
Rudolf I. elected 1273 in Frankfurt
Adolf I. elected 1292 in Frankfurt
Albrecht I. elected 1298 in Mainz
Heinrich VII. elected 1308 in Frankfurt
Ludwig IV. and Friedrich elected 1314 in Frankfurt and Sachsenhausen
Karl IV. elected 1346 in Rhens (near Koblenz) and 1349 in Frankfurt
So yes, an overwhelming majority of royal elections happened in the Mainz-Frankfurt area. Add in the fact that the Archbishop of Mainz was the Archchancellor of Germany, who organized the royal elections, appointed the personnel of the Imperial Chancery, kept the imperial seal and the archives; plus the importance of the Frankfurt trade fair since the 12th century and we have a clear winner.