Ashoka attacked Kalinga in order to further expand the Mauryan empire. Kalinga also had valuable ports which served as a gateway to trade with the far east. Above all, his grandfather Chandragupta Maurya had tried and failed to conquer Kalinga years before, so Ashoka wanted to succeed where his grandfather had failed.
For further reading check the following articles:
|100 Differences Between Articles||UPSC Prelims|
|NCERT Medieval Indian History Notes UPSC||UPSC Mains Questions (2013-2019)|
|NCERT Ancient Indian History Notes UPSC||NCERT Modern Indian History Notes UPSC|