Judging from comments here and from other blogs, perhaps the weather kept them away. According to one out-of-touch poet---is that redundant?---the Japanese killed them all off:
He is waiting to be seen.
In this world I hardly matter.
What goes into the dark
to be seen? Nothing like me.
There is a festival of fireflies
in Muju-gun in August
where people pray for firefly prosperity,
in Korea, since the Japanese
exterminated their fireflies
experimenting with insecticides.
Um, right, well, according to Kim Jong-Gill at the National Institute of Agricultural Science & Technology, environmental changes due to urbanization are to blame.
Only 20 to 30 years ago fireflies were a common sight, but now they have become a nostaligic rarity, highlighted on television shows. In recent years, their natural habitats have been increasingly destroyed and disturbed through environmental damage resulting from the modernization of rural areas and changes in agricultural methods involving excessive reliance on pesticides and chemical fertilizers, as well as street lights that interfere with the ability of fireflies to communicate with each other. Indeed, with all these challenges to its survivability, it is no wonder that firefly sightings are so infrequent nowadays.
You can read the rest of his thoughts on the matter in a 2002 issue of Koreana (.pdf file).
Anyway, I found Muju a cute little town, though there was nothing particularly special about the festival. If you're looking to go, keep in mind that accomodation is prohibitively limited. The few motels in town were all booked, as were all the guesthouses, and we ended-up having to take a cab a few minutes up the road to find anything. If you're without your own transportation you'll be stuck having to make due in town or trying to get a cab either to a motel or to the nearest city. But perhaps if you do go---if you decide to attend the firefly festival during the day---you'll get to meet this interesting assortment of characters: