This is because egg production for hens is like ovulation for all other animals - it just happens as a result of being a mature and fertile female. If you have hens that are reasonably healthy, they will lay eggs approximately every 1.5 days, with more frequency in the spring, summer and fall, and less in the winter.
Egg Production
There are ways to increase egg production, especially during the winter. Egg production is linked to circadian cycles, and the frequency of egg production depends on the hours of daylight the hen sees in the day. When there are fewer hours in the day, the hen produces fewer eggs, with a cycle of up to 3 days long.
There is a very good reason for this, from a biological/evolutionary perspective. During the warm part of the year, baby chicks have a better chance of survival than during the winter. There are fewer predators, and it is easier to keep the babies warm. Food is also more plentiful in the summer than in the winter. Therefore hens have evolved to produce large clutches of eggs when their chicks would have the greatest chance of survival. However, when the eggs are being collected for food and not allowed to hatch, there is no reason to reduce egg production during the winter.
It is very simple to give hens enough light to keep their egg production up: a light in the chicken coop set on an automatic timer will do the trick. You can also go out and manually turn the light off at a certain time each night, but if you forget one night, you may find your chickens stop laying for a couple of days until they recover from their sleepless night. Keeping a light on in the coop while it gets dark outside is also a good way to help chickens become accustomed to sleeping in the coop. They will go inside by themselves, so there is no need to round them up in the evening, but you will need to make sure the door is shut against predators.
Hens are at their peak egg production at about 18 hours of light a day, and some farmers set their light timers to produce that amount of light every day all year round. However, your hens will be healthier and produce eggs for longer in their life if they are allowed to rest, and not required to be at peak production at all times. Therefore it is healthier for your hens to still reduce the hours of daylight they receive in the winter to 12 or 14. Egg production will diminish somewhat, but not as drastically as it would if you did not supplement light at all, especially if you live in an area with very few hours of daylight in the winter. They need to rest in the winter, and you cannot take that period away without endangering their health.
Nutrition
Laying hens have different nutritional needs than roosters, growing chicks, or meat birds. They need more protein than roosters, and they need a calcium supplement to form strong eggshells. Layer mixes have the proper ratio of protein to carbohydrates, and you can buy a calcium supplement such as powdered oyster shells at your feed store. Another option for supplementing calcium is to feed them back their eggshells after you eat the eggs. Just make sure you crush the eggshells thoroughly before feeding them back. so the chickens do not get into the habit of pecking their eggs. Egg-pecking is a serious behavior problem, and if you find a hen pecking her eggs she must be culled from the flock before the behavior spreads.
Many farmers feed their chickens scratch grains. Feeding these grains by hand is a great way to get your chickens used to your presence and to cultivate a relationship with them as farm pets. You can also use scratch feeding time to check your birds for signs of diseases, parasites or injuries, or to see if a bird is being pecked by the others. However, scratch grains consist primarily of nutritionally-poor carbohydrates, so they should not make up a significant portion of your hens’ diet. A good rule of thumb is not to feed more scratch grains than your flock can clean up within 20 minutes.