In my opinion the reason is that all Europe (except Turkey and some other minor Balkan areas) is influenced by Christianity. This religion was for many years the most common factor of all European people, never mind if it was Catholic, Orthodox or Protestant.
Because all Europeans were joined by this common faith, they shared the same God, the same tradition (or Tradition if you like) and for many years also the official language -- Latin. The Christianity comes with the idea of "saints" after whom, to honor them, people name their children.
Because English John, German Johannes, Polish Jan, Italian Giovanni etc. are to honor the same saint, each use of English John was translated to German Johannes because it was the same name of the same saint. In Latin all of names (both first and last) are translated (in modern times you can see this in biology species' names) and this was common practice in all countries in the world (by "world" I mean Europe here). It was not only for nobility, rulers and saints, but everyone, but in fact this "everyone" was for long time the educated people, so nobility and clergy.
In 19th century it was still common practice to translate names of writers, scientists and rich people (bourgeois).
At the moment it is not common practice, I think because of laziness of journalists and other media people, who don't want to look what "Jose" means in English, and use original names in tv and this is no kind of problem if you're watching news that you will forget in an hour.
The other factor is that at the moment, when Christianity is not that important, one can (especially in America) name his/her child for example "Table" or "Jahdadweqw" and this is no problem to translate in the first case (but is this "table" a furniture or a tabular grid of data?), but quite impossible in the second -- although you can use a Google translator now, but not 20 years ago.
So I think that translating rulers names, as well as popes and saints is still a part of tradition. Not translating the names of normal people comes from laziness of (first) journalists, (second) all of us.
In Polish (my mother language), but I'm pretty sure this could be also in peoples who are admired by foreign habits, there is also a feeling, that if something is not ours, it is better (I'd rather say "it was", in 1990s. when the communism has fallen and this was some adoration of Western style of life as only an opposition to Eastern style of life). So when you say "Nicholas Cage" in Polish it's something great, but you never think what "cage" means.
And according to your question, the same "laziness" is now made for rulers/nobility names as for other "normal" people.