When investigating manufacturing chassis with the big frame makers, I was offered 3 tubing treatments with the karting descriptor in brackets:
GBK = annealed (soft)
NBK = normalized (medium)
BK+S = cold drawn and stress relieved (hard)
I believe there’s also BKW (lightly cold worked) and BK (cold finished as drawn) but they weren’t offered.
They didn’t want to use different sizes, available but not preferred.
Heat treatment doesn’t effect the flex, it does effect the Yield Point.
So I’m going to posit a hypothesis (additional to my own post i refound below). A kart is always under some load, through its own weight and the quite substantial (proportionally) weight of its driver (or the drivers lead in @tjkoyen’s case 
). In part of cornering, some part of the kart is entering the plastic region, maybe only slightly, and the load of the weight of the driver is bending it back again (plus the going left and right as we do on most kart tracks). This back and forth hardens the frame over time and also deforms part of it. It’d be interesting to just go round left hand corners for a race length and see if the frame deformed (bent) in that direction. It’d have to be a typical kart surface, not a dirt oval, that relies on the kart keeping a wheel up to be fast.
So by using hardened tubing, designers are moving the yield point to have the kart only just go over it for the given tire engine combo.
Apparently I’ve come to this conclusion before: