I think because of magma composition is very suitable for accumulation of Cu in these magmas.

Porphyry copper deposits are formed by complex geological processes involving magmatism and metamorphism. The main reasons why they are most often associated with calc-alkaline igneous rocks are as follows:

1. Chemical composition of the magma: Calc-alkaline magmas have high contents of alkaline elements (such as sodium and potassium) and calcium, which favor the formation of copper minerals.

2. Temperature and pressure: These magmas form at great depths, where high temperatures and pressures favor the dissolution and migration of metals such as copper and gold.

3. Phase changes: As the magma crystallizes, phase separation occurs, which leads to the concentration of metals in certain zones, creating conditions for the formation of porphyry copper deposits.

4. Tectonic activity: Subduction zones and other tectonic processes often accompany the formation of calc-alkaline magmas, creating ideal conditions for the formation of porphyry copper systems.

5. Associated processes: Hydrothermal processes often occur in these areas, which contribute to additional concentration of copper and other metals.

Thus, the combination of geochemical, thermodynamic and tectonic factors makes calc-alkaline igneous rocks an ideal environment for the formation of porphyry copper deposits.