Why do linguists consider Indo-Iranian a separate subgroup of Indo-European?

Linguists classify Indo-Iranian as a distinct subgroup of the Indo-European family because the Iranian and Indo-Aryan groups share significant linguistic features and vocabulary to a degree that suggests a common origin and development.1

Key evidence for this grouping includes:

• Shared Phonological Developments: Indo-Iranian languages exhibit unique vowel shifts, such as the merger of Proto-Indo-European e, o, and a into a single a sound. They also share specific phonological alternations, such as the development of palatal consonants from original velar consonants in certain grammatical categories.1
• Grammatical Innovations: Both groups share specific grammatical structures, such as the use of an n in the genitive plural ending after certain stems, which is considered an innovation unique to this branch.1
• Common Vocabulary: The groups share significant religious and cultural terminology, including terms for sacrificial rites (e.g., Sanskrit yajña-, Avestan yasna-), ritual substances (e.g., soma/haoma), and shared self-designations like Aryan (Sanskrit ārya-, Avestan airiia-).1

These shared traits indicate a period of common development before the groups diverged into their respective branches.12