How did the Koine language replace many ancient Greek dialects during the Hellenistic era?

The emergence of Koine as a common language (hē koinē dialektos) during the Hellenistic era was driven by the collapse of old political barriers and the establishment of Alexander the Great’s empire in the 4th century BCE.1

As a medium of communication across new urban centers in Egypt, Syria, and Asia Minor, Koine became the standard for business and administration. While it was based primarily on the Attic dialect—due to the enduring cultural and commercial influence of Athens—it facilitated its own spread by absorbing non-Attic elements and undergoing grammatical simplification. This process allowed it to function as a lingua franca, gradually superseding local ancient Greek dialects.12

Although inscriptions confirm its triumphant progress as a administrative and commercial language, some rural dialects persisted as late as the 2nd century CE. The transition was marked by significant linguistic shifts, including phonological changes and the loss of certain classical grammatical features, such as the dual number and the optative mood.1