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Why did Babylon decline after the Seleucids built Seleucia on the Tigris?

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Reconstructed Ishtar Gate at Babylon near Al-Hillah, Iraq, featuring blue walls decorated with rows of yellow and white animal reliefs and a central arched entrance. — Babylon A reconstruction of the Ishtar Gate at the ruins of Babylon, near modern Al-Ḥillah, Iraq.

Babylon’s decline was primarily driven by the deliberate shift of administrative and commercial power to the newly founded city of Seleucia on the Tigris.

After the Seleucid dynasty took control of the region in 312 BCE, they established Seleucia as their eastern capital and a major center for Hellenistic culture. This new city was designed to be an autonomous Greek-style polis, which effectively replaced Babylon as the primary administrative and commercial hub of the province of Babylonia.12

The decline of Babylon was further accelerated in 275 BCE, when a portion of its population was forcibly transferred to Seleucia. By centralizing political authority and trade in a new, strategically located capital, the Seleucid rulers marginalized Babylon, leading to its gradual loss of status and influence in the region.32