Geographers and anthropologists have argued that difficult terrain encourages ethnic heterogeneity, or more precisely, that it can preserve diversity from large, agriculturally supported state-based culture complexes which tend to assimilate as they grow. This is most famously applied to "[Zomia](https://en.wikipedia.org/wiki/Zomia_(region%29)" in highland Southeast Asia, and brought to wider attention in the excellent book The Art of Not Being Governed by anthropologist and political scientist James C. Scott. Scott notes that in addition to highlands, deserts, swamps, and dense jungles can also preserve ethnic diversity. For deserts, we can point to Bedouins in Arabia and the Khoisan in South Africa; for swamps, the Marsh Arabs in Iraq and the Seminole in Florida; for dense jungles, Central African pygmy groups like the Aka or Mbuti or the still minimally contacted groups in the Amazon Rain Forest in Brazil. So, in a certain sense, we'd expect the mountainous terrain of the Balkan Highlands to preserve regional diversity in resistance to lowland state-based cultures, just as we see in Southeast Asia, the Zargos Mountains (where you have Kurds, Turks, Iranians, etc), Switzerland, etc.

Beyond the Greeks and the Albanians, it seems like all the other groups are the result of some post-Roman expansion. Slavs, for instance, didn't come into the Balkans (which at the time were under Byzantine control) until roughly 6th century. The history of the Romanians is very fraught. Turks obviously arrived with the Ottoman Empire, and part of their policy was to bring in other Muslim minorities, which explains how Tatars ended up a minority in Romania. Further, single ethnolinguistic groups got split between religions. Bosniaks, Croats, and Serbs are distinguishable first based on religion. Pomaks and Bulgarians are similar. These religions tend to be related to the large empires that fought over the region through the 20th century (notably, the Austro-Hungarian, the Russia, and the Ottoman Empires). Many regions saw few conquests.

I should note other groups never split based on religion: there are, for instance, Sunni Muslim, Bektashi Muslim, Catholic Christian, and Orthodox Christian Albanians. Slavic groups in the Balkans are also divided by pretty arbitrary lines: Slavic languages tend to form a "dialect continuum" and the difference between these languages tend to be clear from the capital cities. South Slavic languages are specifically mentioned in that Wikipedia article as one of the classic cases of the phenomenon, though Bulgarian is pretty distinct from the others.

A factor is that we default to thinking about Western Europe, when we rarely think of how much ethnic cleansing and assimilation there has been in recent European history. Let's use France as an example. France saw, we think, three or so major conquests: a pre-historic Celtic conquest, the Roman Conquest under Julius Caesar, and the Frankish conquest.

We see that linguistic traces of conquest in France, too. The only pre-Celtic language is Basque spoken in the (surprise, surprise) Mountainous Pyranees in the Southeast. Pre-Celtic languages were mostly displaced by Celtic languages. Actually, all Celtic languages in France were displaced by Romance languages based off Latin. However, Breton, a Celtic language, was brought to what's now the remote Northwestern French province of Brittany/Bretagne, from the Island of Great Britain. Its closest relative is the Cornish Language. The association between Britain and Breton are not coincidental.

However, most of France speaks a Romance language called French, developed from the local dialect of Vulgar Latin. However, this two was a dialect continuum and until very recently, most French people didn't speak French. Those who didn't speak Basque or Breton spoke a variety of romance dialects, with at least two literary standards, commonly known by their separate words for "yes" the Langue d'oïl (oïl being the predecessor to the modern "oui") based around Paris and the Langue d'oc/Lenga d'oc based in the South of France, modern version of which might be called Occitan or Provençal. In the 19th century, Frédéric Mistral even tried to create a literary Provençal nationalism, though this never was elevated to the level of a political nationalism. We tend to think of think of this Langue d'oc as just "a southern dialect of French", but it's probably more similar to Catalan or even some northern Italian "dialects" than to the Parisian-standard of French. The most famous book on this is Eugene Weber's Peasants into Frenchmen; he argues that in about 1870, only about 60% of the citizens of France could actually speak French, and that mass education (only in the Parisian standard, and also teaching a French nationalism), the mass army (again teaching the standard language and inculcating nationalism), and a national economy made possibly by roads and railways are what turned diverse peasants into the modern French. This assimilationist campaign was perhaps most acutely felt in the region of Alsace, which periodic had a German-speaking majority (only being conquered by France in 1689). One 1999 survey found that 43% of Alsatian adults speak Alsatian German, but this is declining in the younger generations. Because of its German heritage, Alsace is also unique in France in its strong Protestant minority and historically a strong Jewish minority (this is where Emile Durkheim's and Alfred Dreyfus's families were from, for instance, and according to one theory, the birth place of Yiddish) though its Jewish population is much less pronounced today.

Or compare the Balkans to the British Isles. There are generally considered to be at least six "indigenous" ethnic groups to the British Isles: the English (descended in part Angles, Saxons, and Jutes), Scots, Welsh, Irish, Cornish, and Manx, each with their own native language and in the Scottish case, two (Scots and Scottish Gaelic). That doesn't count the Romany ("Gypsies"), the Jews, the Irish Travelers ("Pikeys"), Scottish Travelers, etc. who have long made the British Isles their home, nor does it recognize the religious diversity (in England alone, you find Anglicans, Quakers, Methodists, Unitarians, Puritans, Baptists, Presbyterians, etc as well as indigenous Anglo-Catholics and, between 1070 and 1290 and again since 1656, Jews). Much the "internal colonialism" of the government based in London, to use Michael Hechter's term, was against England's "Celtic fringe". Likewise, England did a great deal to tolerate non-conforming (non-Anglican) Protestant churches early on, and since the Reformation Scotland had a separate (Presbyterian) established Church. We tend not to see that diversity today because it's all included in the same country and, for the most part, the same language.

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