Both the Kushan and Gupta Empires became wealthy and diverse due to a combination of factors, but the primary factor that contributed to their prosperity was trade, particularly through the Silk Road.
Trade along the Silk Road brought great wealth to the involved empires as they engaged in the exchange of goods such as silk, spices, precious metals, gemstones, and other luxury items. This trade not only boosted their economies but also promoted cultural diversity as different merchants, travelers, and scholars passed through their territories, leading to the diffusion of knowledge, religions, and artistic styles.