So mother earth was overloaded and she approached Brahmā on account of overloading of these demons. She was feeling too much uncomfortable. And Brahmā is the chief living being within this universe. So when there is need, others approaches Brahmā, and Brahmā approaches Viṣṇu to reduce the overburden. Then He takes His incarnation, and that is stated in the Bhagavad-gītā: yadā yadā hi dharmasya glānir bhavati bhārata, abhyutthānam adharmasya (BG 4.7). Abhyutthānam adharmasya. "When the principles of religiosity becomes increased, at that time, I appear," Kṛṣṇa says.
Just like take for example, when there are too much criminals in the state, unlawful, lawlessness, the state becomes overburdened, disturbed, overburdened. At that time the administrators are puzzled what to do. Similarly when the world is overburdened by atheists, demons, nonbelievers, the world becomes overburdened. Otherwise things will go on nicely.