While it's true that Cd figures have barely changed at all in the past couple of decades, and I'd go as far as to say that as many, if not more, have increased than have decreased, but going as far back as the 60s you'll find Cd numbers were a lot higher.
Tristan is quite correct about size though, the frontal area of modern cars is undergoing a strong upwards trend, thus coupled with near stationary drag coefficients, cars on average experience more aerodynamic drag than from decades past. I'm not quite convinced that things are as bad as they were in the 50s or 60s though (at least, not typical family cars).