Putting them in perspective, the 50s Strat is brighter and more mellow, the 60s have more mid-range, while the 70s Strats take the mid-range to the other level making the guitar sound more aggressive.  Considering how music genres evolved in these three decades, with the guitar gradually shifting from a rhythm to a lead instrument, it's expected that the sound became more present and mid-rangy. The pickups of the three different guitars reflect precisely this in the design.