Well, as I have said, I have problems with some of your posts because in one breath you say you support Penal Substitution and in the next breath you seem to be denying it, or at least, muddying the waters. However, let us proceed.
As you know, I strongly deny that PST leapt like Athene, fully-armed from the brow of Calvin. I gave you a pile of quotations from Church fathers who supported the doctrine, at least
in embryo. Luther spoke of the 'Great Exchange' on the cross (our sins upon Christ; His righteousness upon us) which is certainly substitution. In the book I recommended to you,
Pierced for our Transgressions by Ovey, Jeffrey and Sach, the authors give about 50 pages to the historical pedigree of P.S. and show that its development has been much more gradual than you have allowed. Calvin's work was based (to some degree) on that of Athanasius and Aquinas (amongst others), and developed by Turretine and Owen, for example.
The reason may be that the ECFs were taken up with establishing the Deity of Christ and the doctrine of the Trinity and didn't give the attention to P.S. that they should. The later Roman Catholics were absorbed with the 'sacraments.' In the time of Turretine and Owen, there was a specific need to counter the teachings of Socinius, so the doctrine of P.S. needed to be honed and expounded at that time. I think most theology tends to be reactive.
Well I hope I have given you something to chew on, at least.