I’ve noticed that there aren’t many monarchs who use Emperor as their main title past the Roman era. The only ones I remember are— The Romanovs (Imperator of Russia), Hohenzollerns (Kaiser of Germany), Habsburgs (Kaiser of Austria, Holy Roman Emperor, Kaiser of Austria-Hungary), and the Bonapartes (Emperor of the French). At first, I thought that you could only create an Imperial title if you had jurisdiction over many kingdoms. But then there’s Spain, which encompasses the kingdoms of Leon, Castile, Navarre, and Aragon (+county of Barcelona). Italy had the kingdoms of Sardinia-Piedmont, Lombardy-Venetia and Two Sicilies.

Granted, some monarchs did have imperial titles beside their royal titles (Empress of India, Emperor of Ethiopia). Is it a diplomatic thing, or does it have something to relate with appearances and not pissing off other royals?