Because it's cheaper
Get a book on aircraft structures failing that a general structures book.
Areas to explore:
Mechanical properties
Aluminium is ~1/3 the density, strength and stiffness of steel, in most aircraft structures the forces are basically bending forces with the failure mode being buckling hence the thickness is important with 3rd and 4th powers of the thickness appearing in the calculation of the moment of inertia so for the same weight 1/3 the density gives x3 thickness so 1/3 (stiffness
or youngs modulus) x (3 cubed)= 9 times stronger.
Note that alloying or heat treatment of aluminium (or any metal) does't increase the stiffness only the tensile strength.
Structural design is often about using am ore complex shape to increase the moment of inertia at the expense of higher production in a simple case stringers or a more complex case honeycomb.
Manufacture
Aluminium is a readily available, cheap material which can be easily machined, shaped and joined by riveting by relatively unskilled labour.
Quality control and inspection
Quality control both of the raw material and finished product is relatively simple, likewise insppection in service.
Design
Aluminium is an isotropic material which means the properties are the same in all directions, this simplifies design especially in pre computer days.
Corrosion and rot resistance
Compare other materials eg steel, wood, composites to aluminium using these criteria
This is a gross oversimplification of a very complex topic but hopefully will point you in the right direction.
Remember there is no such thing as an aircraft only a lot of compromises in close formation.