A286 is a high-strength, corrosion-resistant austenitic stainless steel / iron-nickel-chromium alloy that is widely used in aerospace and other high-reliability industries.   

A7075 (Aluminum 7075) is a high-strength aluminum alloy mainly alloyed with zinc (Zn) (with magnesium and copper), widely used in aerospace due to its excellent strength-to-weight ratio.

Because it gives:
High strength
Low density
Good machinability
Proven track record in aerospace structures