The heat-transfer argument
Air conditioning performance ultimately comes down to how efficiently heat moves across a coil surface. Copper's thermal conductivity lets a coil use thinner-walled tube and a more compact fin arrangement to hit the same capacity target — which matters directly for unit size, refrigerant charge, and the energy efficiency rating the finished system can achieve.
Serviceability over the equipment's life
Copper tube can be flared, brazed and re-brazed in the field without special equipment, which matters over a unit's 10–15 year service life when a joint needs repair or a coil needs replacing. This is a practical advantage that shows up in service costs long after the unit has left the factory, not just in the original bill of materials.
Corrosion resistance in real installations
Outdoor condensing units are exposed to rain, humidity, and in coastal or industrial regions, corrosive airborne particulates. Copper's native corrosion resistance — and the option of protective coatings for harsher environments — gives it a service life that aluminium alternatives typically can't match without added protective treatment.
Where the debate is genuinely closer
None of this means copper wins on every metric. Aluminium coils are lighter and cost less per unit of material, which is why some manufacturers use aluminium fins on copper tube, or all-aluminium coils in cost-sensitive product lines. The honest answer is that the choice depends on the application's priorities — and for most mainstream residential and commercial AC systems, copper's balance of performance, serviceability and durability keeps it the default.