I believe it's called typical 70's, 80's, and early 90's unibody construction and design techniques. Once you start looking at the calculations for deflection and load bearing of structural members in these cars, it becomes apparent they cannot take certain impacts, even at relatively low speeds, as they were never designed for it. It's not just a foxbody thing, it's a majority of vehicles that were designed and produced during these times.
I give immense credit to the engineers today that design current vehicles. The regulations and requirements they have to deal with, and the designs they come up with, is just incredible. You only have to view a couple offset crash tests from 80's vehicles compared to current vehicles and it becomes painfully obvious. The advancement of computer simulations and engineering tools have also had a huge impact on this as we are able to simulate real world scenarios into designs with ease, compared to years of research and trial and error in the past.