Because it's in the mid-range of the current generation, as opposed to the high-end of the previous generation. Newer tech, means it uses less power, and thus puts out less heat, but still offers around the same performance.
Or, it can offer more performance, but at the same power use as the previous generation. Compare HD4870 and HD5870.
Or, it can offer more performance, but at the same power use as the previous generation. Compare HD4870 and HD5870.