I often make buying decisions based on a coin's popularity. As an example, I will buy a coin like an 1839-O quarter eagle for stock because it is popular and I know it will sell. But I might pass on a rarer coin like an 1862-S quarter eagle because it is not a popular issue and it will be a harder coin to sell. This got me to to thinking: what makes one coin popular and another unpopular? Certain 20th century series are popular with collectors because of a strong nostalgia factor. I would imagine most of the collectors who focus on Lincoln Cents or Mercury Dimes remember collecting them as a kid and the sense of accomplishment that they get from completing a set is an act of closure that extinguishes the nightmares they felt as kids about filling those pesky 1909-S VDB Cent and 1916-D Dime holes.
The nostalgia factor does not really apply to gold given the fact that circulation for these coins ended in the early 1930's. There are certainly some collectors who can remember being given an Indian Head quarter eagle for the holidays by their grandparents or aunt and uncle. But I'm willing to bet that the majority of gold coin collectors are not working on a set of Charlotte half eagles because it rekindles pleasant childhood memories.
The word "promotion" gets a bad rap in numismatics. Yes, there are naughty promotions where worthless modern trinkets get hyped and sold to unsuspecting people for multiples of their true value. But in the better sense of the word, coin promotions can turn formerly unpopular series--like Type One Liberty Head double eagles--into popular ones. The key to a coin promotion is that it has to be sustained and it needs more market participants than the first wave to regenerate its initial success(es).
I mentioned the Type One double eagles series in the last paragraph. One of the most brilliant coin promotions of all time was the S.S. Central America.. The marketing group that owned the coins not only was able to sell them, they were able to generate enough new interest in this denomination that it impacted all Type Ones, not just the few dates that were included in the hoard.
A coin that is historic is always going to be popular. What represents "history" to be may not be what represents history to you. But I'm almost certain we can both agree that a gold coin produed in the 18th century--the first decade of the operations of the new U.S. Mint--is clearly historic. This is one reason why a coin like a 1795 half eagle or a 1799 eagle, while not truly "rare," is still always going to have a very high level of demand among collectors.
Other coins are historic due to numismatically significant factors. In the first paragraph I mentioned the 1839-O quarter eagle. This is a coin that is fairly common in lower grades yet I still really like the issue and will buy nearly any problem free example I can find. Why? It is a first year of issue coin, it is a one year type and it is a visually interesting issue in that it has the short-lived Classic Head design paired with the obverse placement of the mintmark. These factors give it broad appeal and it is one of the few New Orleans quarter eagles that would be of interest to a non-specialist.
Another factor that makes a coin popular or unpopular is its design. A coin like a High Relief is extremely popular because it is beautiful and its design appeals to virtually all collectors. "Plain Jane" coins like three cent nickels and liberty head nickels get little respect from collectors because, to be frank, they are not especially attractive designs.
There is an element of geography with certain types of coins that makes them popular. Dahlonega gold coinage has a large following in the Atlanta and North Georgia area due to having been produced in this part of the country. This isn't to say that a collector from New York City might not become a major collector of Dahlonega gold but the majority of interest in these coins tends to be in the south.
A factor in determining the popularity of a coin has to do with the availability of good reference material. Without tooting my own horn too loudly (OK, at least not blaring it...) I'd like to think that part of the reason that Southern branch mint gold is popular has to do with the fact that I have written easy to use basic reference guides that are updated every few years, inexpensive to purchase, readily available and easy to transport to coin shows and auctions. This might be a coincidence, but the two least popular 19th and early 20th century gold issues (Philadelphia and San Francisco mints) are the ones that I just happened to not have written books about. At least yet...
Size has become an important factor in making gold coins popular or unpopular. Double eagles are more popular than gold dollars mostly because they are much bigger. People like bigger coins and new collectors feel more justified in spending $3,000 for a big, hefty double eagle than they do for a small gold dollar. As the age of collectors continues to climb, it seems inevitable that even more people will shy away from small coins; if only because they will have trouble seeing them.
These popularity factors are just a few of the reasons why certain coins are popular and others are not. If you ask many collectors why they specialize in a certain area, the reason may be hard to determine. Often, these individuals collect "what they like" and there is no clearly definable reason; it's just a gut instinct that, say, tells them that they should focus on Proof gold dollars or Gem Saints. And that's the beauty of rare coin collecting.