Why was Patliputra considered as the most culturally rich city?

Pataliputra is the ancient name of Patna, the present day capital of Bihar. Pataliputra has had many names throughout its continuous history - Pataligram, Pataliputra, Palibothra, Kusumpur, Pushpapur, Azeemabad, Bankipore, and the current Patna. The name Pataliputra may have derived from the Sanskrit Word pattan, meaning port, or from the Word patali, meaning Trumpet flower, in honor of Patali, a legendary queen. Two of the city's other names, Pushpapur and Kusumpur, both mean City of Flowers. Ancient Chinese texts refer to Pataliputra as Pa-lin-fou.

The ancient city of Pataliputra was stretched out along the south bank of the Ganga River. The Ganduk River empties into the Ganga from the north at Pataliputra, and the Sone enters the Ganga from the south, some miles to the west of the city. A smaller river, the Punpun, also enters the Ganga from the south at Pataliputra. Because of the city's location near the confluence of three large rivers, trade has always been an important part of its economy