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The terrain. Most of southern India is covered by the Deccan Plateau.
According to Wikipedia,
"The plateau is located between two mountain ranges, the Western Ghats and the Eastern Ghats, each of which rises from its respective nearby coastal plain, and almost converge at the southern tip of India. It is separated from the Gangetic plain to the north by the Satpura and Vindhya Ranges, which form its northern boundary."
Perhaps the greatest barrier to the south is the Nilgiris Mountains
"The Nilgiris district is one of the 38 districts in the southern Indian state of Tamil Nadu. Nilgiri (English: Blue Mountains) is the name given to a range of mountains spread across the borders among the states of Tamil Nadu, Karnataka and Kerala. The Nilgiri Hills are part of a larger mountain chain known as the Western Ghats. Their highest point is the mountain of Doddabetta, height 2,637 m. The district is contained mainly within the Nilgiri Mountains range."