Why is the Indus Plain a tropical desert climate?

The Indus Plain is located in Pakistan in the northwest of South Asia, and it is influenced by the airflow of the south branch of the westerly wind in the northern hemisphere in winter. After the vernal equinox, the direct point of the sun moved to the northern hemisphere, and the south branch of the west wind began to retreat. The area controlled by the southwest monsoon extends from southeast to northwest, but the south branch of the west wind retreated late here in the Indus Plain, and the southwest monsoon could not reach it. The virgin forests in these places were destroyed, and the climate gradually dried up, forming a tropical desert climate.