Closing of ancient silk route and northern/northwestern travel routes would have led to a situation where these ancient tribes travelling into India for trade or bought as slaves or came as invading armies were not able to escape/travel or meet other tribes, etc. This ancient thought could very well be true as we know that even "Alexander the Great", did not move beyond present day Punjab, as in ancient times moving inside beyond Punjab into India could have created a trap where ancient Punjab was filled with rivers and marshes and in the north were Himalayas, so escape for these people was almost impossible.
Later Armies like that of Scythians did travel into India but again with time the ancient connections with Eurasia, China, Mangolia,etc., were closed, time and time again. Then there was a continuous process of Dhasanskar or Dha ka Sanskar(sanskar is a religious word), burning the dead and not burying the dead, thereby removing the ancient blood and tribal links of these tribes. What remains today are the words/names/languages, etc., that these tribes and people still carry forward, which still link them to these ancient roots.