What is the main reason the 160-metre and 80-metre bands tend to be useful only for short-distance communications during daylight hours?

The D region is the lowest layer of the ionosphere and becomes highly ionized and active during daylight hours due to incoming solar radiation. Unlike higher layers that effectively refract radio waves, the dense D region absorbs lower frequency signals, particularly those in the 160-metre and 80-metre bands. Because these low-frequency radio waves are heavily absorbed before they can penetrate upward to reach the refractive E or F layers, daylight communications on these bands are primarily limited to short-distance ground-wave propagation. Once the sun sets, the D region dissipates rapidly. Without this daytime absorption layer, 160-metre and 80-metre signals can pass through to the higher layers and travel much further via skywave propagation.