Hello there! The Nationalist Government offered a brief explanation when they officially designated Chongqing as China's wartime capital on September 6, 1940 (though Chiang Kai-shek had been there since November 1938):

Szechuan (Sichuan), known as heavenly palace in ancient times, is gifted with majestic landscape and abundant resources. Strategically located along the Yangtze River in the southwest, Chunking (Chongqing) has been selected as the temporary seat of government at which the nation began to reestablish itself in the War of Resistance. Three years have elapsed in our preparations for what may take place in the future. With a common hostility of the enemy, the people of Szechuan have given generously and have been resolute in their determination to lay a solid foundation in the War of Resistance and to undertake the task of national reconstruction. At present, the position of the nation's wartime capital is more consolidated than ever, as it has become the pivot in military, political and economic affairs. After the termination of the war, it will become the center of reconstruction in the southwest. The people are unanimously agreed that efforts will be directed to rebuild the nation. Accordingly, Chunking is hereby designated the nation’s wartime capital. The Executive Yuan is directed to supervise the responsible agencies and take into consideration the organizational system of Hsiking (Xiqing?) so as to realize the long-range plans, fulfill the wish of the people, and prepare for great celebrations. [1]

Now here's a more sophisticated explanation:

Chongqing possessed undeniable strategic benefits as the site for a new base of operations. The city was situated on a narrow peninsula that formed the confluence of the Jialing and Yangtze rivers, affording it a direct line of access to the coast via the river system. Built atop a rocky edifice, it was flanked on each side by sheer cliffs that provided strategic vantage points of the surrounding area and formed a natural defensive barrier against enemy incursions. These unique topographical features, combined with the city's advantageous position along a major transportation artery, entered into Chiang Kai-shek's tactical calculations when he issued the official relocation order in November 1937 [actually 1938!]. [2]

I hope this helps! :)

[1] Hsu Long-hsuen and Chang Ming-kai (compilers), History of the Sino-Japanese War (1937-1945), transl. Wen Ha-hsiung (Taipei: Chang Wu Publishing, 1971), 17.

[2] Edna Tow, "The Great Bombing of Chongqing and the Anti-Japanese War, 1937-1945," in The Battle for China: Essays on the Military History of the Sino-Japanese War of 1937-1945, ed. Mark Peattie, Edward Drea, and Hans van de Ven (Stanford: Stanford University Press, 2011), 258.