Question

why a fuse wire is made from an alloy of lead and tin.

Answer 1

Alloys (e.g. tin/lead) are used due to their 'eutectic' action -i.e. the resulting combination of good conductivity (due to the tin) with a low melting point due to the lead).

Answer 2

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The fuse wire is made of tin or alive tin and copper because these materials have a melting point lower than that of normal connecting wires and devices. This helps melt the fuse wire before any other component and saves them from burning.