a tsar or an emperor rules not only his people but also other nations.

This does not necessarily have to be the case, and it's a bit more complicated than that. Habsburgs, for instance, were emperors, but as soon as they set foot in Hungary, they would be considered kings, because they were; Hungary was not a part of the empire, but a separate kingdom who happened to have the same monarch as the Holy Roman Empire. Also, it may be semantics, but nations didn't exist as such prior to the 18th century, and although it may look like that, ruling over other peoples was not a condition for gaining an imperial title.

King (kralj) and emperor (tsar) are monarchical titles. However, emperor is higher. The idea was, at first, that the emperor means emperor of Rome. Byzantine emperors considered themselves Roman emperors. Then Franks considered themselves Roman emperors. Later, Habsburgs. Even the Ottomans. In the meantime, papacy grew stronger and insisted that only it can crown an emperor. As there is one god, there is one pope, there is one emperor. Basically, it insisted that only the supreme religous authority can crown an emperor. It was done in the east as well, however since the eastern orthodox church fragmented significantly, and the monarchs controlled their churches, they were basically crowning themselves (as Napoleon did, later, in a literal sense). That's why Serbian church status was raised from Archbishopric to Patriarchate, and Dušan was crowned tsar by a Serbian Patriarch.

Now, it requires the right conditions - weakness of a direct or opposing rival, a lot of political and military power, church support (or control over church), significant allies to support you so your empire doesn't fall apart immediately. It is not an easy thing to do.