Well, firstly, historians aren't very clear about the southern limits of Chandragupta's empire. Some say he did conquer the southern kingdoms, others say the southern kings were his vassals, but he didn't include them in his empire. Tamil literature mentions the arrival of the "Moriya" at about the right time (3rd century BC), but we don't know whether these were the Mauryas, or someone else.

Chandragupta mostly focused on extending his empire northwest across the Gangetic plain and then the Indus plain. He started in the east from Patliputra (modern Patna), which was the center of the Nanda dynasty he displaced. From there he seems to have followed pretty much a straight line northwest across the breadth of the subcontinent, extending through India, Pakistan and Afghanistan. He didn't stop until he met the Greeks and took back what Alexander had taken from Taxila and Gandhara. According to Strabo, Selecus Nikator ceded all of Alexander's lands in modern Balochistan and Afghanistan to Chandragupta, and retreated to Iran.

So in Chandragupta"s time, the Mauryan empire was this broad band extending southeast to northwest across the entire subcontinent. He did absorb parts east of Patliputra (Bengal), but we can sort of see it as an extension of the series of alluvial plains extending west to east - the Indus system, the Ganges, and then the Meghna-Brahmaputra Complex in Bengal. This is the largest and most fertile chain of alluvial plains in the world, and this was the backbone of Chandragupta's empire.

Regarding conquests south of the Vindhya mountains, historians are unsure whether he actually conquered the lands or whether they simply accepted vassal status.

His son Bindusara seems to have spent his time consolidating rather than extending the empire, but the conquests began again under his grandson Ashoka. So perhaps the better question is why didn't Ashoka extend direct rule to the southern tip of India.

The traditional reason offered is that Ashoka gave up war and turned Buddhist. Ashoka got a bloody start, killing 3 of his brothers to gain succession after Bindusara's death. Then he set about extending his grandfather's empire. The first target was Kalinga, which was a very large and very rich kingdom right on his border, in modern Orissa.

But Kalinga resisted fiercely, and the war dragged on. So Ashoka didn't defeat Kalinga so much as he completely wiped it out. There was so much slaughter that Ashoka got sick of the bloodshed and had a change of heart.

So after the defeat of Kalinga, he converted to Buddhism, took a vow of non-violence, and spent the rest of his life spreading Buddhism. It was Ashoka's missionaries who first took Buddhism to China and Japan, to southeast Asia, to Sri Lanka, and to the Tarim basin in the southern Caucasus.

So one way to look at it is that Buddhism saved south India from the Mauryans. Ashoka was bloody enough in the beginning to take the fight south, but he never fought again after Kalinga.