I think all of that had something to do with the idea of Christendom, specifically the Christian monarch. With the Fall of Constantinople, the only reigning 'Eastern Roman' monarch became the Russian czars (note, not Moscow, but Russian.) This goes along with the fact that 'Russian Orthodox' does not refer to 'Russki', ie Russian ethnicity, but Russiye - under the Russian Imperial crown. Now, that's been over about 90 years.
With Rome (Old Rome) the last Roman Emperor abdicated back in the 19th c. (last of the Holy Roman Emperors.)
So - there are no Christian monarchs ruling over vast areas today than Queen Elizabeth I of Scotland (known in England as Queen Elizabeth II.) She is, of course, officially Anglican (of a respectable High Church English Use variety), though Presbyterian when in Scotland.
I think that only leaves:
Catholic kings: Albert II of Belgium, Furst Hans-Adam II of Liechtenstein, Grand Duke Henri of Luxembourg, Prince Albert II of Monaco, King Juan Carlos I of Spain.
Protestant kings and queens: Margrethe II of Denmark, Koningen Beatrix of the Netherlands, Norges Konge Harald V, Konung Carl XVI Gustaf, King George Tupou V of Tonga (ie, Lutherans and Methodists.)
Of those, only Spain and Denmark have lands beyond their home countries (Juan Carlos is also the titular crusader 'King of Jerusalem'.) Only the British monarchy has any sense of being Christian with rule over large areas of the world. And, so far, restoration of Orthodox monarchs has been - well, 'duds'.
All the rest of the world monarchs are Muslim, Animists or some Eastern religion.
I suppose Russia has the best chance of restoring a monarchy. The Greeks - little to no chance (their last King was no Emperor, and most surviving claimants to descend from the Emperors have competing claims - and are usually either Roman Catholics or Muslims.)
That's a fun little are of interest to obsess over...