Here's why:
* Horcruxes are made from dark magic: They are created by splitting the soul and anchoring it to an object. This is a very dark and dangerous act. The Invisibility Cloak is a magical artifact, but its magic doesn't stem from dark arts.
* The Invisibility Cloak is a family heirloom: It was passed down through generations of the Potter family, suggesting it's not a creation based on dark magic.
* Horcruxes are not intended for protection: Their primary purpose is to preserve a piece of the soul, even if the body is destroyed. The Invisibility Cloak, while helpful, is primarily used for protection and stealth.
The Invisibility Cloak's magic works differently: It utilizes a powerful form of enchantment that bends light around the wearer, making them invisible. There's no mention of soul splitting or dark magic involved.