Why was the magnesium put in a copper basket? (Why not some other metal?)

According to the reactivity series, it is known that a more reactive metal is able to displace a less reactive metal when both of them are present together.

So as per this series, magnesium is more reactive than copper. Hence, when magnesium is placed in a copper basket then copper is unable to displace magnesium as it is less reactive in nature as compared to magnesium.

Therefore, no chemical reaction will take place. That is why, magnesium is put in a copper basket.