Is there a reason why or is it just the quality of the materials?

Stamped = Data pits are a physical, unalterable thing on the disc. CD-R = Pits are still physical, but they are on a die layer that can fade. Some discs are more resilient (blue, gold) than others (cyan). CD-RW = Pits are (mostly) bistable for practical purposes, but will degrade to a neutral state on a long-term basis.

CD-Rs use an organic dye which fades (or in the worst case the bonds holding it on can delaminate).

They hold up much better if stored in a dark place. A spindle on a shelf that sees sunlight is the worst case and they can become unreadable in years.