Anonymous quotes:
"---but the real puzzle is why only these frequencies?"

Shortwave propagation has always been highly variable. Selective fading
is the rule. So much so, that frequency diversity has proved effective
to improve reliability even though 2 or more frequencies must be used
for only one channel in but a single direction.

Hit or miss communications makes ionospheric radio obsolete for
practical purposes in an age of satellites and optical fiber cables.

For amateurs, hit or miss is part of the fun.

Best regards, Richard Harrison, KB5WZI