Like George- I think stainless is for cosmetic reasons. Many years ago I was Making Kart Frames and we did build a Frame out of Stainless and to the same tube dimensions of the normal Tubing we used. Weight was a bit higher but the handling was way off. It also suffered tube breaks in places that normally break but a lot earlier in the life of the frame.

Admittedly not a fair trial as this frame was punished to the limit and it was not bike use. I dare say that with development we could have got the handling better- stopped the tubes breaking and got the weight down but we stayed with what we knew and understood.