Tiberius reduced the number of mints, made Lugdunum the only mint in the western part of the empire to issue silver and gold coins, and restricted the mint in Rome to issuing bronze coins. Caligula reversed that by ordering the Roman mint to produce gold and silver coins again, and restricting Lugdunum to bronze. All bets are off during the chaos of the Civil Wars following Nero's suicide. As the Flavians took control, the primary resposibility for minting gradually returned to Rome. Coins are often dug up in Europe to this very day.
Popularity is the most subjective quality of all.
It has nothing to do with the condition the coin is in, how many were minted, or how many exist. It refers to the economic rule of supply and demand. Julius Caesar is the most famous Roman, and every collector wants one of his coins. The demand is high, so the price is also high. Later coins are less popular, easier to find, and comparatively less expensive.