I think it's for practical historical reasons. Frequency is really a nicer property because, unlike wavelength, it doesn't change as the wave goes between media that are at rest with respect to each other. With sound waves the frequencies are low enough to be measurable even with pretty simple methods. The frequencies of visible light are far too high to measure directly with the sort of devices that were available at the time this descriptive convention got started. On the other hand, light wavelengths are directly measurable by looking at interference patterns from gratings and other old-fashioned devices.
Mike W.
(published on 12/24/2015)
