The one word answer: Sunspots
The somewhat more detailed answer is that the higher frequencies are better during the day, but when the sun goes down, and the sunspots are over other parts of the planet than ours, skip on the higher bands goes away.
Things are slowly getting better with the increase in sunspot numbers with this new solar cycle, but we're not at the peak yet. That's roughly 5 years away, and then the numbers will start to decrease slowly but surely. That will make the lower bands (160-30 meters) more desirable, especially during the winter when the nights are longer.
20 meters, even with low sunspot numbers, almost always shows some DX during the daytime. FT8 will be the best mode for such conditions (sorry, digi-haters), but CW will also work pretty well. 17-10 (sometimes even 6) will be at their best for next few years, but they're still normally daytime bands. As always, there will be exceptions so don't completely write those bands off at night.