I'm not sure you can point to just one reason. It's not a new phenomenon. And there are probably a lot of factors that goes into it. I'll discuss a few but I'm sure others can point to more.

Athens and Sparta dominated existing contemporary works of history that has had a lot of influence in the modern day.

Obviously, the History of the Peloponnesian War by Thucydides chronicles the war between Athens and Sparta. Thucydides was famous for being relatively neutral in his work. Although, whether he was successful or not is an open topic of debate he certainly meant to present Sparta in a fair light. And could be very critical of Athens. So, the resulting work presented two fairly balanced forces at work.

This is a hugely influential work that is still read today. There was never a point where it was no longer read.

But before that Herodotus's The Histories has a large role for both Sparta and Athens. The battle of Thermopylae is written about there with the brave Spartans held off a far larger Persian force for three days. The battle of Marathon is written about there with the Athenians defeating the Persians. These are stories that people who know nothing of ancient history still know today.

Herodotus mentions many Greek states and Persian satraps. Really what doesn't Herodotus talk about? But some of the stories that still capture the public imagination about the Persian Wars come from him and deal with Athens and Sparta. I think it's impossible to understate how important Herodotus's legacy is in how we think about the ancient world before the Peloponnesian war.

Xenophon's Constitution of the Spartans also has a lot of influence on how the culture has subsequently been viewed. While he was from Athens he wrote admiringly of the Spartan constitution and people.

The contrast between the two peoples has been compelling to writers since Xenophon or before. The military state of Sparta epitomizing something of a manly ideal compared to the more decadent Athens, which epitomized more of an intellectual ideal.

And, of course, many (but by no means all) of the most famous Greeks are somewhat associated with Athens. Herodotus wasn't from there but he did spend time and present his work there. Socrates, Plato, and Thucydides were all from there. The most well known playwrites presented their work there. We could go on. Even in the ancient world Athens was seen as the birthplace of philosophy and long, long after it had any particular political importance men would go there to study. The future emperor Julian was one.

And, of course, they were for a time genuinely the two superpowers for a certain period of time. Other city-states (like Corinth and Thebes) had great military success as well. But Athens and Sparta began growing in power at roughly the same time and eventually led to a long off and on war. That's memorable.