Why are my chickens sleeping in their nest boxes rather than their roosts?
It's difficult for us to say exactly what might help without being able to inspect your coop, but I can give you some pointers that may assist you in solving the mystery.
First, make sure your roosts are higher than your nests. If your nests are higher, your chickens will instinctively go for the highest place they can reach--do make sure it isn't too high for your chickens to easily reach, though! Second, make sure your roosts are comfortable for your birds. Although we tend to picture birds roosting on branches in the trees, most chickens actually prefer flat roosts to round branches. They are ground birds. Flat roosts (like a 2 x 4, wide side up) allow them to roost comfortably, and in cold weather their toes stay covered and warm as they sleep. Last, make sure your roosts aren't made of metal. Sometimes people have the idea to use a metal pole as a roost, thinking it will be sturdy and easy to clean. However, they are hard for a chickens to grip, and can cause frostbite in cold weather.
If your coop is arranged correctly and your birds still aren't using the roosts, they may just be young and not have it figured out yet. If your birds are silkies, sometimes silkies just do prefer to sleep in a heap on the floor together like puppies, and sometimes birds with large crests like Polish can have difficulty seeing how to get to their nice roosts. If that's the case, a careful trim of any feathers blocking their vision could help.
But most other breeds, even if they are young and haven't figured it out on their own yet, will eventually "get it" and use their roosts for sleeping. To help them get the idea earlier, you can try lifting them up to the roosts every night for a few days until they go up on their own.