Question

Why did trade flourish under the Abbasids?

Answer 1

The Abbasids used something similar to a banking system. People invested in long distance trade and goods were bought on credit.

Answer 2

Trade flourished under the Abbasids at first because they moved the capital of the Muslim empire to the newly created city of Baghdad in central Iraq. The city was located on key trade routes that gave the caliph access to trade gold, good, and information about.
I hope my answer helps you!