Actually, I think it’s directly related to the improved usuability of high ISO values: in both cases, how far you can amplify the signal is dependent on the S/N in the shadows (the lights usually take care of themselves).
So if, with the same raw developer, an old camera gives good results at ISO 800, and a new one can go to 6400 with the same perceived quality, you’d have a gain of about 3EV in the shadows (very rough estimation).
However, do not just compare results for in-camera jpegs, or developments done with different (versions of) a raw developer: that technology also evolved… And that probably accounts for part of the perceived gain.