The simplest and, indeed, the only certain answer is because he can get in that way.
People have been puzzling over this question for quite some time. The influence of “A Visit from St. Nicholas” may have made Santa’s entrance by chimney part of America’s standard Christmas lore, but Santa’s connection with chimneys actually dates back several centuries before the poem.
We can see the association already in a charming seventeenth-century Dutch painting by Jan Steen titled “The Feast of St. Nicholas.” It depicts St. Nicholas’ Day (the giving of Christmas gifts still takes place in several countries on December 6): A little girl is delighted with her gifts, while her brother bewails the fact that he has received nothing. Most interesting for our question, though, in the background an adult and two toddlers are looking up the chimney, amazement on their faces. It is clear that St. Nicholas—or at the very least the gifts he brought—came down the chimney. (You can view the painting in the Rijksmuseum in Amsterdam.)
The chimney motif may have originated with the early story about St. Nicholas tossing bags of gold through a poor man’s window to prevent him from having to sell his daughters into prostitution. It is not so great a leap to have him toss the money down through the chimney instead, especially since in northern climes people probably would have shut and barred their windows in the cold of December. And, in fact, a later version of the story has St. Nicholas using the chimney in precisely this way when he found the windows locked on his third visit. Besides, as early as the seventh century, stories give the saint the ability to teleport himself any distance instantly, the kind of magic many a parent has offered as an explanation of how a portly man with a huge bag of presents gets down and up narrow chimneys.
Some scholars have looked to pre-Christian pagan lore and practice to find the origins of Santa’s chimney entrance. Hearths, for example, were thought to house spirits or gods who were critical to the prosperity of the household, and fire itself was held sacred. It would not be surprising, then, for a myth to develop in which bounty is bestowed by way of the sacred openings associated with these powers.
Another explanation traces the origin of Santa’s chimney entrance to the primitiv
e belief that supernatural creatures like elves and fairies enter houses through chimneys. It is no coincidence, in this view, that “A Visit from St. Nicholas” calls Santa “a right jolly old elf.”
Other elements of pagan religion and of life among the people of northern Europe can reasonably be connected with Santa’s chimney. However, historical evidence tying these elements directly to Santa’s unconventional mode of visiting us is lacking. Lovers of history may be frustrated, but lovers of myth and lore still enjoy engaging in speculation.
—reprinted with permission from Were They Wise Men or Kings? The Book of Christmas Questions by Joseph Walsh, Ph.D., professor of classics and history