He explained a long time ago. The main problem is in available raw material, graphite prepregs used to construct a layup. Currently available graphite comes from the aeroindustry, it's a HM graphite stiffer than what would be ideal for tennis racquets considering comfort (playability is another issue, HM graphite performs fine there). To be able to drop the RA yet have enough firmness and durability in a frame, Paul combines prepregs with particles at orientation of 0, 30 and 60 degrees, which recquires more layers than usual simple 0+90 degrees construction which typically creates a racquet around 70 RA strung with the minimum number of layers. Still, given a number of needed layers to meet required firmness and durability, there's a flex limit for this design construction, I suppose.
The obvious problem is raw material market. Aeroindustry has high demand and it seems they dictate both what is being manufactured and available capacity. My speculation is that manufacturing of lower modulus (softer) graphite prepregs, as a custom order, might have a drastically higher price, completely unrealistic for smaller custom orders. I suspect this is the reason why tennis manufacturers, even the biggest ones, avoid making anything with the strung RA in 50's. The only racquets made such are either very light racquets (with its durability under question mark), or through using composites of graphite plus softer material layers (or braided fibres like Wilson does it) to get some additional flex, but either such rarely go to 50's RA strung for some reason.
Besides all said, on a shorter run with the stiffer frames it's actually easier to play better tennis because of more forgiveness from a bigger sweet spot and hence easier power. I guess there's space to go flexy at the throat but stiffer in the hoop and that this is the way to go. If I was a racquet designer I'd allow myself to use a composite layup for the throat, but a 100% graphite hoop with some clever layup design like Paul does, letting the flex issue mostly to the throat. My opinion is that flex in the throat is most responsible for comfort, there are reasons for this.
Other than this I believe Paul's designs are a mastery. I also believe if he had on his dispositon raw material of modulus of his choice, the very same designs plus this raw material would create racquets which would make players who had them to never look back or further for anything else. Firmer feel you get at least on some Angell's is a direct consequence of HM raw material he has on disposition, I'm quite sure about this.