Plus, lower frequencies generally have an easier time traveling long distances, especially over saltwater (and at night, hence the first part of your response).

And to answer the question about why we can't use lower frequencies on planes, part of the answer lies in antenna length, which is related to wavelength. Lower frequency = longer wavelength, which in turn means larger antenna (yes, I know this is a very broad generalization, and also not the complete/only reason). Look up the antenna required for 1Mhz.