I can think of a number of reasons why.
Novelty......only kid on the block to have one.
Laziness....I and some others stopped shooting blackpowder because we are too lazy to clean our revolvers every time.
Availability.....sometimes blackpowder is hard to find depending on where you live.
Practicality.....you can shoot longer without fouling.
Safety......less likely to flash over and less likely the the powder on the table will be set off by a spark.
Indoor ranges.......you could now shoot inside because of no smoke.
Economy.....smokeless powder is cheaper because you need fewer grains.
Hunting.....no need to worry about rain soaking your powder as much.
Home defense.....you can load the revolver and leave it not having to worry if the powder got wet over time.
Self defense.......ditto except one would not have to worry about sweat.
Now the self defense and home defense are just the added bonuses along with the others, especially if it is the only handgun one has at the moment, but there is another reason for the home/self defense mention.
In some areas (hopefully Heller may change this) one cannot own a modern handgun or carry it around, but there are sometimes gaps in the law about "antiques" or muzzloaders.
Also, some states prohibit people under 21 or under 18 from possessing or at least buying modern handguns, but in some of those states they may buy and own cap and ball revolvers. A smokeless one could be the better alternative because they can trust it to fire without worrying about humidity or water affecting the powder.
And of course there are the people in England and Scotland which still want to shoot handguns but don't want to deal with blackpowder for the numerous reasons.
