The reasoning is actually pretty straightforward. The predominant religion in Russia is still Russian Orthodoxy which still follows the old Julian calendar.
While the official calendar that’s in use daily is the new Gregorian calendar, religious holidays are celebrated according to the old calendar. In the old calendar, holidays are shifted by about 13 days, which means that Christmas and Easter come on different days from the familiar Western traditions.
Russian Orthodoxy has been in place for over a thousand years. With the exception of a 70 year break during Communism. Three generations grew up not knowing anything about Christmas or its religious traditions.
Religious symbols were replaced with secular motifs and explanations. The evergreen Christmas tree became a symbol of New Years, a gift giving saint transformed into Father Frost “Ded Moroz” who left presents for children under the tree. With religion no longer the focal point, traditional Christmas foods slowly began to disappear from the holiday dinner tables. Or, more likely, they were simply adapted into similar dishes but lost their meaning along the way.
Fortunately, during the past two decades Russian Christmas food traditions have regained their proper place in society. More and more families are observing Christmas and enjoying traditional Christmas foods. These days you can even find authentic Slavic Christmas food on restaurant menus across the country.
There are lots of recipes for Russian Christmas food that you can find online, but the best and most precious ones are still passed down from generation to generation.