A fruit cake involves a wide variety of dried fruits and a good volume of sugar, both of which, were absolute luxuries in the earlier times. Along with these rich ingredients, booze of any kind was also required to be added, keeping in mind the cold weather. If these weren’t enough, a wooden oven that burned with the help of woods was also necessary to bake this cake. Due to all of this hustle involved, this cake was booked for special occasions like Christmas when all of the hard work seemed worthy.
Source: Thanks https://timesofindia.indiatimes.com/life-style/food-news/why-are-some-foods-specially-eaten-on-christmas/photostory/72963978.cms