Becasue, as stated before, it has the highest conductivity of all metals. But you want to know why that is, right?
Silver sits in the middle of the transistion metals approximately 1/2 way between the noble gasses and the alkali metals. In column 11 of the periodic table, all of these elements (copper, silver, and gold) have a single s-orbital electron outer shell electron (platinum does also, in column 10).
The orbital structure of the electrons of these elements neither has a particular affinity to gain an electron or lose an electron toward the noble gasses that are heavier or lighter, because they sit 1/2 way in between. In general this means that it doesn't take much energy to knock an electron off temporarily, or add one temporarily. The specific electron affinities and ionization potentials are varied, and concerning conduction, having relative low energies for these two criteria is somewhat important.
If those were the only criteria, than gold would be a better conductor than silver, but gold has an extra 14 f-orbital electrons underneath the 10 d-orbital electrons and the single s-orbital electron. The 14 f electrons are due to the extra atoms in the Actinide series. With 14 extra electrons apparently pushing out on the d and s electrons you'd think that s-electron was just sitting out there 'ripe' for conduction (hardly any energy was necessary to bump it off), but NOOO. The f-orbital electrons are packed in, in such a manner, that it causes the atomic radius of gold to be actually SMALLER than the atomic radius of silver -- not by much, but it is smaller. A smaller radius, means more force from the nucleus on the outer electrons, so silver wins in the conductivity 'contest'. Remember, force due to electric charge is inversely proportional to the square of the distance. The closer 2 charges are together., the higher the force between them.
Both copper and platinum have even smaller diameters; hence more pull from the nucleus, hence more energy to knock off that lone s-electron, hence lower conductivity.
Other elements with a single s-orbital electron sitting out there "ripe for the conduction picker to come along", also have lower atomic radii (molybdenum, niobium, chromium, ruthenium, rhodium) than silver.
So, it is mainly where it sits -- where 'mother nature' put silver in the periodic table, that dictates its excellent conductivity.