Because every single species of bird on earth has a pre-determined number of daylight hours when they start producing sexual hormones and another when they stop. This is what photoperiodism is all about: the bird's endocrine system reacting to the number (period) of light (photo) hours. When you keep a bird on the same number of daylight hours and the same diet every day of the year, their endocrine system goes out of whack and starts producing sexual hormones all the time.
When we first started keeping parrots as pets, we thought that they could be kept the same way that we keep dogs, cats, horses, etc. - namely, what we now call a 'human' light schedule (meaning NOT following the sun). To put the icing on the cake, we knew NOTHING about their diets and fed them sunflower seeds, peanuts and table food crap with the inevitable consequences: hormonal (read aggressive parrots; parrots that plucked and self-mutilated; birds that died VERY young, etc. Then we thought we had found the solution by keeping them at the 12L/12D (12 hours of light and 12 hours of dark) schedule because -again, in our ignorance- we thought that parrots were all tropical and that, if in the tropics they lived with this schedule, they would be perfectly fine if we did the same. The problem with this was that not all parrots are tropical but also that even tropical parrots are photoperiodic (and we now know from studies that their bodies actually register the difference of 20 minutes there are between the seasons) AND that they use food availability and weather as triggers (in the tropics, you don't have summer and winter but you do have rainy and dry season which determines food availability) while, in captivity, there are no monsoons or food shortages of any kind so they still produced sexual hormones all the time. Fastforward a few years and we realized that even tropical birds will 'revert' back to using photoperiodism as their main trigger if they are exposed to a temperate climate schedule (meaning big differences in the daylength between summer and winter). But even the solar schedule doesn't work well UNLESS you expose them to the different spectrum that happens during twilight - thus, the strict solar schedule with full exposure to dawn and dusk. I go one step further and give them low protein during the winter and higher protein as well as a richer diet during the spring and summer.