It was designed to carry to operate from very rough "airstrips" which is a very optimistic term for "a paddock that the farmer hopefully mowed recently and if you're lucky, they also removed most of the bigger stones".

I also imagine in the postwar WW2 antipodes, steel was a lot easier and cheaper to access, as well as work.

That was a prototype.

Update: I guess the final design also used steel.

> The pilot is above both the engine and the load, and is surrounded by a steel tube truss for maximum safety.

Steel alloys have better fatigue properties than aluminum. Many of us in aerospace would happily use a corrosion-resistant steel if not for the weight.