This was because the Edo Shogunate controlled islands south of Etorofu and had guards stationed on those islands to prevent incursions by foreigners.
In 1930, 8,300 people lived on Kunashiri island and 6,000 on Etorofu island, and most of them were engaged in coastal and high sea fishing.
Kuril's defense was entrusted to Kita Chisima and Minami Chisima "Fortress", Imperial Japanese Army defensive organization along with the Imperial Japanese Navy with command in Sapporo and Nemuro.
The Second Article of the Treaty provided that the boundary line was demarcated between Etorofu (Iturup) and Urup, and "all the Island of Etorofu belongs to Japan, all the Island of Urup and the Kuril Islands to the north of it (Urup) belongs to Russia" in the Japanese text (underlined by the author).
Secondly the Kuril Islands are geographically on the chain of volcanos from Kunashiri to Shumushu.
Kunashiri and Etorofu are excluded of the Kuril Islands abandoned by Japan in the Peace Treaty of San Francisco.