Analysis of information sources in references of the Wikipedia article "Ali Mech" in English language version.
Their adherence to the caste system kept them away from establishing roots with the local population. The Vedic religion which the Aryans brought with them never took strong root in the local people of the region. This might be one of the factors that led to a good portion the Mlecchas population ( such as the Mech tribe in the north according to Tabaqat-i-Nasiri) to cooperate with Muslim conquerors identified by the Aryan (Vedic) Hindus as Yavanas (originally Sanskrit word meaning polluted outsiders/aliens
Even Richard Eaton (Eaton 2014: 118) observes that "Muslim converts in Bengal, were mainly from the Rajbanshi, Pod, Chandal, Kuch and other indigenous groups that had been lightly exposed to Brahmanic culture.