Because Denver didn't make any small dates in copper. They had transitioned to zinc and they were all supposed to be zinc. 1 single 1982-D small date copper has been discovered (left over copper planchet in the hopper with a million zincs) so that 1 single coin is worth a lot of money, because, it is an off metal (copper instead of zinc) and there is only 1 of them. The US minted 17 billion cents in 1982. The small date is more desirable than the large date (Philly, not Denver) in copper and zinc, but only in the highest grades, because they made billions of them. The surviving zinc Philly 82 small dates are more desirable because there are less of them. They had problems with the new zinc cent, and less of the 1982 small date Philly zincs survived the process. There are many more large dates than small, and the Denver smalls are not as "valuable" as the Philly smalls.
