Why do PC bus frequencies often end in multiples of 33MHz?
Once you have one subsystem operating at a multiple of a particular crystal frequency, it's convenient to use the same basis crystal for generating other frequencies. This changes the line of inquiry to "what was the first use of 33MHz in computers?", which may be some 386 processors or the PCI bus.

It's possible that the first supply of 33MHz crystals was for some radio purpose, but googling is not suggesting a ready answer.