Glen D. of QJ.net writes, If you're wondering why Pandemic Studios and THQ draped their upcoming Nintendo Wii game in a 1970s theme, the game's creators gave a simple explanation: the first Destroy All Humans! Big Willy Unleashed was set in the 1950s, while the sequel had a 1960s template. It should be logical then, that this one be set in the 70s.
But aside from chronology, it was stated in the Destroy All Humans! Big Willy Unleashed blog that the 70s have a unique charm to it because of the socio-political atmosphere of the region. A lot of changes took place, and they wanted to share to the gamers of today a more accurate depiction of the era.