Question

why did samudragupta decided not to annex the kingdom of dakshinapatha

Answer 1

the rulers of dakshinapatha were a set of 12 rulers.they surrendered to samudragupta  after being defeated in the battle and samudragupta allowed them to rule again.

Answer 2

Since iron age, the term Dakshinapath was used to describe the southern highway connecting Pataliputra to southern Godavari kingdom. ​Samudragupta was a powerful king who wanted to expand his glory and power by making a powerful kingdom. He attacked the rulers of Dakshinapath but later he restored them to their kingdoms after they accepted his suzerainty and overlordship.  

Samudragupta's decision was driven not only by generosity but also by strategic motives. He estimated that in the absence of proper transportation link between the north and south India, governing such a wide kingdom would be impossible. From his centre of power in the north, efficiently managing the far flung Dakshinapath would have been really difficult. That is why he gave up his idea of directly controlling the Dakshinapath.