The Iga region of Mie Prefecture and the Koga region of Shiga Prefecture are famous as the birthplace of the ninja, and are referred to in the "Omi Yochi Shiryaku", an Edo period geographical journal, as "Ninja (shinobinono) Iga Koga," meaning "Ninja," and the ninja were represented as "those from Iga and Koga".
The two provinces, also known as "Koui-Ichi Province", are bordered by rolling hills and are neighbors from north to south, and they are in close proximity to Kyoto and Nara.
Surrounded by the Suzuka Mountains to the east and the Kasagi Mountains to the west, the mountainous region was a convenient place of exile for the powerful of the time, a strategic point for east-west traffic on the Yamato Highway and Tokaido Highway, and a militarily important region as well.
The answer to the question of why ninja and ninjutsu were born in the Iga and Koga regions can be found by visiting the villages of the ninja.