Iirc by the Hohenstaufens the role of the ecclesiastical electors were to be ArchChancellors for Italy, East Francia/Germany, and Middle Francia/Arles/Lotharingia.
The German post had more or less settled on Mainz; the Middle post was disputed by Trier (for Lotharingia) and Vienne (the primate bishop of the Kingdom of Arles); and the Italian post was variously on Aquileia, Besanzon, Cologne, Milan, Salzburg, and Vienne, depending on imperial relations with the Italian nobles and Popes.
With Italy becoming a battleground between Pope and Emperor, and the disintegration of former Lotharingia and its successor Arles, North Western Archbishops became a lot more "reliable" as Electors allowing Mainz, Trier, and Cologne, to secure it permanently.