Tire design has improved over the years, and not just in tread design. Tires wear down much more slowly and provide more grip than they have in the past. Many manufactuerers of tires have also switched from natural to synthetic rubber.
The basic design of a rubber compound tends to be a trade-off between grip and durability; softer rubber will provide more grip, but isn't as durable, so it doesn't last as long. A harder rubber compound will last longer, but doesn't cling to the road as well as a soft compound would. Tire manufacturers try to make new tire compounds which increase both durability and grip, as well as for other features favored in production vehicles, like ride comfort, noise reduction, and the reduction of rolling drag (which can save a small amount of fuel).