Why were nomadic pastoralists viewed with suspicion in ancient India?

Because they did not fit into the settled agricultural framework

Throughout the subcontinent's history, diverse populations have existed, whose social practices remained unaffected by Brahmanical ideas. When mentioned in Sanskrit texts, they were often portrayed as peculiar, uncivilized, or even animal-like. For instance, forest-dwellers, who relied on hunting and gathering for their subsistence, were described in such terms. The nishada category, to which Ekalavya is believed to have belonged, is an example of such groups.

Additionally, nomadic pastoralists were viewed with suspicion as they did not easily fit into the settled agricultural framework. Those who spoke non-Sanskritic languages were labeled as "mlechchhas" and looked down upon by some. Nevertheless, there was also a mutual exchange of ideas and beliefs between these diverse groups. This is evident in certain stories found in the Mahabharata, showcasing the complex nature of their relations.