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.
Collecting Roman coins is a great way to learn about ancient Roman history, culture, and politics. It is an amazing and satisfying experience to hold a 1,800 year-old coin in your hand.