Simple - tungsten carbide is one of the world's hardest and most scratch resistant substances. Compared to other jewelry materials, its scratch resistance and general resilience is extreme. For example, it's 2x harder than steel, 4x harder than titanium, and a whopping 10x harder than gold. Keep in mind that hardness is a measure of scratch resistance.
This means Tungsten Carbide rings keep their amazing finishes for far, far longer. Often times it's hard to tell a tungsten ring that has been worn for 10 years from a new tungsten ring once you give it a good clean with a cloth! It's that long lasting.