Silver is a better conductor than gold and in Integrated Circuit package where exposure to air should not be a concern Gold is used. Any particular advantage of using Gold over Silver?
Apart from oxidisation and tarnishing, silver migration is also a serious concern, to the point that alloys are required to mitigate this.
However alloying silver also means that the advantageous increase in conductivity with respect to gold is somewhat lost. This might be the reason why silver has not taken over gold for wire bonds.
OTOH: nowadays, palladium-plated copper wire bonds look very promising, with their performance very close to gold wire bonds (even better: the copper-aluminium intermetallic products formation is just 1/100th of that of the gold-aluminium system - the co-called "purple plague"), and their cost below silver wire bonds.
Look at this report published by NASA about copper wire bonds. They might even evaluate them for space flight.
Gold doesn't corrode. Silver will oxidize and tarnish, reducing conductivity and potentially breaking the wire. Gold doesn't react significantly with any common component of air or the plastics used in ICs, even at elevated temperatures, and so it's the material of choice for bond wires.
There is a large body of knowledge about bonding with gold wire. Much is known about process and reliability. Gold wire can be stored in air and remains bondable for many months after purchase. It is the most convenient type of wire for setting up a process the first time. Some gold wire types are extremely malleable lending themselves to being bent into almost impossible wire shapes. Some packages with advanced looping are sometimes only possible with gold wire.