Why is Modern Persian described as a koine language?

Modern Persian is described as a koine language because it developed from the contact and synthesis of multiple dialects, specifically Middle Persian and Parthian, with additional influences from other Iranian languages such as Sogdian.1

In linguistics, a koine is a language that emerges when different dialects of the same language interact over a large region, adopting elements that are widely recognized while discarding those that are not. By the 9th century CE, Modern Persian emerged through this process of integration. As the national language of Iran, it subsequently exerted strong influence over other Iranian languages, serving as a common medium of communication and literature across a vast geographic area, including parts of Central Asia and the Indian subcontinent.213