Just like any other objects that collect dust if not maintained well, diamonds get dirty. Dust collects over time and the sparkles on the diamonds disappear. Diamonds are lipophilic; oil and grease can collect on the diamonds’ surfaces fairly quickly. With the combination of oil and grease, dust will start piling up on diamonds and that is how diamonds lose their sparkle as light is not able to reflect on the diamonds anymore.
However that does not mean that they can never sparkle again. All it takes is a nice clean so that the diamonds can regain their sparkle again…