Probably because it doesn't last long, and as a seasonal drink is only in high demand around the holidays. It's just one of those things that people don't habitually buy throughout the year. If there were a market it'd most likely be sold year-round.
Eggnog... now there's something I haven't had in a while. I tend to mix it up with a splash of this and a splash of that - rum, brandy, whiskey, Kahlua, amaretto, and whatever else I can locate, as complicated as possible (within reason). Cuts down on the thickness and makes things a little more fun