It’s really simple!
It’s because the hearth revolves around the sun!
During it’s movement around the sun and because of the angle of the rotation axis, the length of days changes during the year. In winter the sun comes from a lower angle (in the northern hemisphere) and days are shorter; in summer sun is more vertical and days are longer.
During autumn the hearth rotation axis and the sun align so that days get shorter and shorter. On the 21st of december (for the solstice) the rotation axis and the direction of the sun kind of align (it’s a simplification but it helps keeping it simple and I didn’t want to introduce the idea of declination) and the day is at it’s shortest.
With the beginning of the winter days start to lenghten but it’s only by december 25 that the lenght is sensibly higher therefore many a religion see in this particular day the symbol of rebirth.
As you see it’s really about astronomy and physics
