I think one thing that must be considered is how leadership changed. The last "The Great" that I can think about in European history is Catherine the Great. Being a autocratic ruler with very broad authority to do what she wanted, the same with Frederick the Great whom lived at the same time. Then this guy named Napoleon comes out of nowhere and becomes a name.

What is special about these people is that they enjoyed rather broad authority to do what they needed. This would change during the 19th Century when there was a strong push for Constitutional Monarchies that Britain was already enjoying. So with Constitutional Monarchies, you see less power in the hands of the Monarch and more in the government as a whole. You start to see the rise of politicians as leaders, say Lord North or William Pitt, as major political movers. So non-monarch leaders come up in positions of temporary power that are able to be powerful enough to be known either by their name or the time period they served.

So, I argue, and I hope there are people whom will disagree, that the lack of epithets has more to do with the change of political climate to a more democratic and egalitarian air.

In England, you tend to see this epithets falling away after the Conquest. This is conjecture on my part, but I think it may have to do with the fact that the Normans established a dynastic succession (whereas the Saxon kings were dynastic with a caveat, that the Witan had some influence on who specifically became King, especially in the event of Kings dying without issue), and Kings, Saxon, Dane and other took their thrones by force. While there was a degree of that still happening in later years, essentially all claimants to the throne after the Conquest claimed inheritance in a line from William the Conqueror. I would argue that the development of the numbering system, in England anyway, had at least something to do with legitimizing rulers in a time when Kings could still be unseated by a stronger claim or stronger forces. You might, for example hear Edward VI called the sixth of that name but actually he wasn't, since there were like three before we even started counting that way, but he was the sixth who could claim descent from the Conqueror (even if indirectly and sometimes in combination of right by conquest). Even in cases where the succession moves laterally (such as from Elizabeth I to James VI and I), the new monarch is chosen by right of having the best blood claim to a regent legitimate monarch.