Answer:
Japan's geographical condition.
Explanation:
Japan and the rest of asian continent was separated by the pacific ocean. During medieval time, both the Chinese and the Mongols did not possess enough naval technology to make the transportation to Japan ran smoothly.
On top of that, during the first two trial of Invasion to Japan, Mongolian soldiers faced heavy defeat. The geographical shape of Hakata Bay (a point of entry for Invaders who wanted to enter Japan) was really hard to penetrate. Causing Chinese and  Mongolian soldiers to suffer heavy casualties before they even set foot on the land.
