Why are ancient Iranian languages believed to have been more mutually intelligible than later Iranian languages?

It is widely accepted that the degree of mutual intelligibility among ancient Iranian languages was significantly greater than that found between Middle Iranian languages. While the exact extent of this intelligibility remains uncertain due to the limited nature of surviving sources—such as the religious poetry of Zoroaster in Avestan versus the official royal inscriptions in Old Persian—linguistic evidence points to a more unified foundation during the ancient period.1

This higher level of mutual understanding is attributed to the fact that ancient Iranian languages shared a more consistent phonological and morphological structure derived from their common protolanguage, Common Old Iranian. Over time, the Iranian languages underwent a progressive simplification of their complex Indo-European grammatical systems, moving toward more analytic structures. As these languages evolved into the Middle and Modern periods, they diverged further through varying developments in consonant clusters and morphological changes, which reduced the shared linguistic features that had previously facilitated easier communication between geographically proximate groups.2341