Here's why:

* Phosphorus (P) usually forms compounds with a maximum oxidation state of +5.

* Chlorine (Cl) typically has an oxidation state of -1.

To satisfy these typical oxidation states, the compound should have a formula closer to PCl₅ (phosphorus pentachloride).

There are other possible compounds containing phosphorus and chlorine, but P₂Cl₈ is not one of them.