The most likely reason is that mountainous regions help a people resist being overwhelmed by ruling elites or expansive national groups.
The Balkan mountains, for example, helped preserve the various Slavic peoples during the era of Ottoman and Austrian domination. The Carpathians helped preserve Ukraine. The Czechs and Slovaks were also protected to an extent from the Germans and Hungarians by mountains. The Alps allowed the Swiss to resist Hapsburg conquest.
