I think that older cars had low CR because the engineers wanted to leave room for abuse in the lower end. Higher CR is harder on the lower end. A driver of a high CR engine needs to be mindful of lugging such an engine. Trouble is the I6 is a torque monster that will tolerate low RPM lugging. The same is true for truck power plants - the engineers are mindful that the operator is likely to encounter circumstances that could cause him to lug it . . . pulling a load on a grade in traffic can easily lead to lugging, even with a mindful operator armed with a tachometer.
There is also a need for higher octane fuel (which an overly cheap operator may not want to provide) . . . leading to pinging with otherwise properly set timing . . . which abuses the top end.
Low CR is a safe design construct for reliability in a world without a tach - let alone mass air flow and oxygen sensors and an engine management computer to dynamically adjust mixture and valve events . . .
