Tampering with the control surfaces of an aircraft, to "improve" the handling has to be outside the scope of what is "normal" for GA.
The STOL kits, including Robertson are STC'd in the normal category. They add to the capability of decently capable aircraft by adding complexity to increase performance. Probably most purchasers of new Cessnas would not want to pay for this increased capability, the same way that many car purchasers are content with the "factory"version.
The modification to droop the ailerons makes the R STOL equipped Cessna very similar to the deHavilland Otter and Twin Otter, among others who droop ailerons as a part of their normal type design.
As for the cuffed leading edge modification to the Cessna wings, It was well regarded by Cessna such that they changed their type design to make a similar cuff standard on most 100 series in the early '70's. The 150/152 were not included, as they became poor spin trainers with a cuffed wing.
Many airframe manufacturers not only support STC modifications on their basic aircraft, but will buy and resell, or recommend them to owners. Cessna sought me out to buy many copies of an STC (as a "part") I hold on their aircraft, and thereafter asked my in increase my approved model list to add Cessnas which I had previously not considered approving. They offered to buy the STC from me in the whole, but did not offer me enough money, so I still have it.