Why is gold preferred over silver for bond wire?
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.