I've wondered as well why they didn't move in that direction. Penske's choices over the years revealed an inventive mind that thought along lines others apparently didn't consider. In this instance it's right there, you can see it when you ordered the wheels, the wheels came cast for 4 lugs. Penske and Donohue came from racing platforms that used wheels held on either by knockoffs, pins, or probably 4 lugs, so the concept wouldn't have been a huge leap.
It is safe to say the overall trend of original ideas he and his team developed were often so simple that in retrospect I can imagine the faint echoes of a collective "DUHHHHHH" ebbing through the cosmos. A short list of looking at what the rulebook doesn't say would include: the wide filler neck and long fill tube for the TA Camaro to increase the fuel capacity without breaking the letter of the rulebook, the tall gravity feed fuel tower for fuel stops, the original 4 lug incarnation of Minilites in '70, using non-floater rear axles on '69 Camaro and relying on super strong axle shafts and repetitive teardowns and inspection, using 917 like brakes front and rear on the roundy-round Matador in '71, elliptical rear springs on the '69 Cams and '70-'71 Javs, horizontal traction shocks on the '69's- similar to AMC's articulated though rigid "torque link" system, the vinyl roof on the Camaros, and so on.
In looking at the team's approach to building a car balance comes to mind first and then details aimed at saving time for events that are considered down time are as much a part of a competitive edge by saving a few seconds in the pits with 4 lug wheels as would having a powerful mill and complex rear suspension a la` Mustang, and thus would have been 1970's "Unfair Advantage" for Penske.