Someone more experience correct me if Im wrong, but to have much more sidecut would actually inhibit the structrual integrity of the ski. if you had 140 tips and tails, and a 75 waist, it would be so torsionally weak that it couldnt physically handle the turns youud make on it.
Thats just my guess, but I dont really know shit
The simple reason why twins are generally longer radius is that for big air, park, even pipe riders need stability on take-offs and landings. If u have a short radius ski, which are generally a twitchy or nervous ski, u have more chance of screw up on landings and take offs. I am referencing this from some pros and skicompanies. U don't want squirrelly.
The same goes for riding in pow. U want to be in control of the ski not the other way around.
In some places u are not allowed to get your instructors or coaches certificate unless you are on a "normal mid radius ski". No slaloms or twins allowed. This forces you to demonstrate the kinda technique the examiner wantes to evaluate.
Since there is always someone who wants a quicker turning ski - ski companies make them. We have a lot of choices today.