From the NIST WWV FAQ:

"Multiple frequencies are used because shortwave propagation varies
with many factors, including time of year, time of day, geographical
location, solar and geomagnetic activity, weather conditions, and
antenna type and configuration. In general, the lower frequencies of
2.5 and 5 MHz are best during nighttime hours, the higher frequencies
of 15 and 20 MHz are better during daytime hours, and 5 and 10 MHz are
probably the best compromises overall. The 5, 10, and 15 MHz
transmissions are at higher power than the other frequencies."

MW