Because it is a proof coin! The designation "proof" has to do with how the coin is struck - they are made for collectors and special sets - struck from special dies, etc. Although they were not meant for circulation, sometimes they end up there - someone breaks up a proof set they were given and spends them.
I did not want to mislead anyone into thinking this one had not been in circulation - but as stated - it is a "proof" with very few circulation marks showing.