Why is the origin of Indo-Aryan languages debated among scholars?

The origin of Indo-Aryan languages remains a subject of scholarly debate primarily due to the lack of definitive archaeological and textual evidence to support competing theories.1

While a majority of scholars support the migration thesis—positing that Indo-Aryan speakers moved into the Indian subcontinent from an external homeland in Central Asia—this is not universally accepted. The debate is characterized by the following points of contention:1

• Migration vs. Indigenous Origin: Some scholars argue that Indo-Aryan languages originated within the Indian subcontinent and subsequently spread outward. Proponents of this view point to the lack of clear archaeological evidence for a large-scale migration or conquest, as well as the physical continuity observed between Harappan and post-Harappan societies.12
• Linguistic Evidence: The migration theory is supported by linguistic analysis, which suggests that the early Vedic system is not equivalent to Proto-Indo-European. Methodologically, it is considered simpler to reconcile the vowel systems of Indo-Iranian languages with an external origin rather than assuming the complex internal shifts required by an indigenous-origin model.1
• Interpretation of Ancient Texts: Evidence from Middle Eastern texts (c. 1450–1350 BCE) containing potential Indo-Aryan loanwords remains subject to varying interpretations, and Sanskrit sources cited as "memories" of an ancestral homeland are considered by many to be inconclusive.1

Ultimately, the issue is complex, and definitive, probative evidence remains elusive.1