Because the Persian civilization had Indus as its border in modern day Pakistan and didn't spread further. Our ancestors from the Indus valley civilization, traded and seeded civilizational structure from the Persians which is why the west of Indus bank is classified as Indo-Persian.
Persian Civilization predates the IVC
IVC and Harappa were our ancestors which handed civilizational structure to other parts of India. The Indo-Aryan migration as it is believed also happened through what today is Pakistan to India.
In turn, IVC is a successor to the Persian civilization which was a successor to the Babylonians.