Aluminum vs Steel
Aluminum is 1/3 the weight of steel and has a third of the (tensile) strength. Aluminum frames, on the other hand, can be constructed with a larger gauge and bracing to withstand the weight of heavier people.
When carrying the cot, shelter personnel appreciate the lower weight of an aluminum frame. They also like not having to worry about rust (corrosion) ruining the frame's appearance and eventually causing it to break (bending or buckling).
Although aluminum corrodes like steel, the corrosion process results in a thin film of aluminum oxide forming on the metal's outside, effectively galvanizing it and protecting it from further corrosion. The metal looks to have just lost its former lustrous sheen, but no more corrosion or damage to the frame occurs.
There is no paint to crack or scratch on aluminum-frame cots, exposing the metal surface to dampness and causing corrosion. These cots are unaffected by minor scrapes or blows to the metal surface.
Because aluminum corrodes only slightly, an aluminum-frame cot can last for years without danger of rust and corrosion causing the frame or legs to collapse over time.
Aluminum light-weight cots may weigh half as much as steel cots for the same amount of volume.
"Why not just use a lighter gauge of steel to produce a lightweight cot?" you might wonder. Light-gauge steel would deteriorate more quickly to the point of failure if exposed to outside moisture and temperature change.