But even where the day/night variation is less pronounced, lots of groups still have a December-solstice-related holiday. The Hindu holiday Makara Sankranti is a post-solstice celebration, marking the beginning of the solar month after the solstice, so it usually falls in January. It’s not as widely known among non-Hindus as Diwali or Holi but it’s still pretty important, and apparently very ancient.
The ancient Greeks had the Haloea or Festival of Poseidon at the winter solstice, which in fact helped inspire the Roman Saturnalia celebration.
AFAIK pretty much any culture that has a seasonally-based calendar and an agricultural heritage has some kind of celebration linked to the shortest-day solstice. This is true even for cultures which also have a religious calendar that’s not seasonally-based, such as the Muslim calendar. Iranians, for example, who are overwhelmingly Muslim, celebrate the solstice holiday Shab-e Yalda. Ancient Egyptians celebrated the births of the deities Osiris, Horus, etc., during their calendar’s “epagomenal days” after the winter solstice.
Off the top of my head I don’t know of any longstanding cultural tradition in the northern hemisphere that doesn’t have any annual celebration at all related to the winter solstice. (In the southern hemisphere, of course, the December solstice falls in summer.) However, I agree that there are comparatively few such holidays that are widely recognized in modern popular culture.