Comment by tdeck

It's because when the term was first used, newer electronics to generate those frequencies had only recently become available.

The HF band is 3-30 MHz. Maritime navigation was operating on 500kHz (.5MHz) for most of the 20th century because that was what Marconi's alternators used at the beginning of the system. These machines produced the carrier frequency mechanically by rotating a disc with lots of tiny magnets on it at high speed.

https://en.wikipedia.org/wiki/Alexanderson_alternator