It's a question that I always thought about. I can understand for Mayence, Trier and Cologne, being archbishoprics their position as electors is justifiable, Bohemia for its importance in the Empire, but for example, why Brandenburg or the Palatinate ? Why such insignificant nations in the greater Empire were chosen ? I think I read somewhere that it was linked to their position before the Golden Bull, but still, why were those specific nations chosen to elect the Emperor ?
Since the origins of the electors date back to the Frankish empire, the roles of Mayence, Trier, Cologne, and Palatinate can be explained with their geographical situation in the core of the Frankish Empire. So it was less about actual political power, but about derived dignity.
Research indicates that initially, we know about four laudatores: The archbishoprics of Mayence and Cologne accompanied by Palatinate and Brandenburg as representatives of the secular power. The sovereign of Saxony was added to prevent stalemates. As far back as Eike of Repgow's »Sachsenspiegel« (early 13th century), the sovereign of Bohemia was explicitly not included because, »he is not a German«. Fun fact: Keeping Bohemia out might have been a smart thing because on the electors’ first real election in 1257, the seven electors did produce a stalemate because the Bohemian king voted for both of the candidates, causing an interregnum period until 1273.