Strength resistance from initial vertical impact force from the steel front bracket and pliable compliant vibration absorbing resistance from the alloy bracket. Offers a less harsher ride and reduced chance of the vehicle altering direction due to impact with road imperfections.
Strength and flexibility on the same wishbone. If they used both as steel they would offer no "give"and are more susceptible to sheer forces on impact with potholes etc.
If they used both brackets in alloy they would bend through flexing and fracture prematurely over time.
They offer each other support through both material benefits mutually but greatly reduce the down sides of using just one primary material.
The same reason the body of the lower wishbone body is steel and the upper wishbone body is alloy. They compliment each others rigidity and compliance.
If you take a look at the whole Alfa suspension set up you will notice from top to bottom:
Steel bottom wishbone, alloy suspension fork, steel bodied shock absorber strut, alloy upper wishbone, steel spring, alloy upper mount.... So stiffness, rigid strength with "give"and compliance in one whole unit with a slightly less corner weight too.
There's is always a good reason but it's also why Alfa Romeo seem to have the uncanny ability to make a saloon,estate, coupe or hatchback handle like a sports car yet still give a smooth ride with excellent communicative feedback when others marques can't do it in the same manner of compromise
It's why we buy them and love their handling characteristics?
There is always a trade off though and corrosion between the materials is exactly just that... A small price to pay when it works so well though.
Another reason is safety in the event of collision from non uniform directions of operating design. Alloy offers less crush resistance on impact so will buckle rather than sheer like steel. Therefore an alloy bracket has more chance to keep the wishbone in the wheel arch area upon unnatural impact rather than sheering off and penetrating the firewall/foot well or even worse, another vehicle or pedestrians.
Hope this answers your question