Because it’s proven. Aryans were Caucasoid races only.
Otherwise, why only Caucasoid races in Europe & Asia speak common Indo-European languages? Why Chinese, Japanese, Africans, Semitics (real or ancient Semitics were Afro-Asiatic— not Caucasian or Caucasoid), Australoid races of Melanesia, Papua New Guinea, Pacific, Indonesia, Melanesia etc. don’t speak Aryan languages?
Why Indo-Aryans of India having more genetic, linguistic & cultural similarity with Caucasoid races of Pakistan, central-Asia, west-Asia & Europe — than to their neighboring China/Asian races?
For your information, there are still pure Caucasoid looking IE speaking races in subcontinent.
And race-mixing still happening , between those Aryans & other non-Aryan races.
Which gives rise to mixed features– that is the proven base of the theory behind Nordic/Caucasoid identity of Aryans.
Indian nationalists & patriotic peoples deny it, because it shows unpleasant aspect of their society, history & culture. They have hard times accepting the fact that Caucasoid Aryan race in India got reduced due to mixing with non-Caucasoid or non-Aryan races.
Still, pure or archaic non -Caucasoid races can be found in India & they speak non-IE languages–which stands as the greatest proof.
See DNA test reports of a average Pakistani & Indian. The Pakistani sample has zero Australoid /Dravidian that’s why he looks very much Caucasoid or Nordic-skull shape. Give this Pakistani boy less melanin (blond hair & paper-white skin), he will perfectly resemble a white German. Many of you already know how those Kalash, Pashtuns & Kashmiris with light eyes & light hairs look like.
Now Indian sample. The more Australoid inputs– the more deviation from Caucasoid features. With white skin or blond hair, he’d still look different from a European Caucasoid.
So, you can see why Nordic or Caucasoid feature is seen as standard of Indo-European or Aryan race. No wonder today’s Pakistan, Afghan, Tajikistan was the epic-center of Ancient Vedic Aryans who spread all over Asia & Europe & mixed with different native races.