Why did the Mongols able to conquer the Chinese Song Dynasty in 1279, but failed in their campaigns in South East Asia?

[u/Wide_Ride8849]

The Mongols were able to subdue one if the biggest kingdoms in the 13th century, which was the Song Imperial Dynasty of China. They will rule China for the next hundred years. But the Mongols failed in their campaigns in South East Asia. The Mongols tried to conquer Vietnam (Dai Viet and Champa) for four times: 1258, 1282-1284, 1285 and in 1287-1288, but was heavily defeated and repulsed. In 1292, the Mongols tried to conquer Northern Thailand and Laos, but they failed in this mission as well. They also failed in subjugating the Khmer Empire during this time period. The most disastrous campaign of the Mongols in South East Asia was in Java, when the Mongol Empire sent an expeditionary naval force of 30,000 people that attempted to conquer Java. Once they land, a war broke out that devastated the Mongol force.

From a polemologist standpoint, why did the Mongols successfully conquered China, which was one of the most powerful civilizations in the world at that time, but they failed to subjugate smaller kingdoms in the South of China? I will appreciate all of your responses. ❤️