Here's why:
* The Balkan Peninsula is characterized by a complex system of mountain ranges, including the Dinaric Alps, the Balkan Mountains, and the Rhodope Mountains.
* These mountains are interspersed with valleys, plateaus, and coastal plains.
* The presence of numerous mountain ranges with high peaks and steep slopes makes the terrain predominantly mountainous.
While other words like "rugged," "diverse," or "challenging" also apply to the Balkan terrain, "mountainous" best captures the defining characteristic of the region.