Here's why:

* Covalent crystals are characterized by a continuous network of atoms held together by covalent bonds. This means the atoms share electrons in a vast, three-dimensional structure. Examples include diamond and silicon dioxide.

* Phosphorus exists in various allotropes (different structural forms), but none of them are covalent crystals.

* White phosphorus (P4) is a molecular solid with discrete tetrahedral P4 molecules held together by weak van der Waals forces.

* Red phosphorus is a polymeric structure with chains of phosphorus atoms linked by covalent bonds. However, these chains are not continuous in a three-dimensional network like in covalent crystals.

* Black phosphorus has a layered structure similar to graphite, where layers of phosphorus atoms are held together by weaker van der Waals forces.

Therefore, while phosphorus exhibits covalent bonding in its different forms, it does not form the continuous network structure characteristic of covalent crystals.