Svaswamishaktayoh svaroopopalabdhihetuh sanyogah
Shaktayoh: of their powers
The recognition of the form or nature of Purush and Prakriti is the cause of their union.
The reason for Purush to combine with Prakriti is to experience the actual nature or form of both. An individual takes a body to go through the experiences and it is only through the body that it can carve its way back to its elemental state.