It seems like there are a lot of frequencies for different system buses that end in 33 or 66MHz. Is there a particular reason for this?
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.
The exact number is not 33.0000MHz, it is 2^25 Hz, in lower power CPUs, it is 2^15. It is easier to implement a power of 2 counter than other arbitrary number.
Edit: After digging into the PCI spec and the 486 datasheets, I didn't find any hard requirement for clock accuracy. The PCI spec allows operation from DC to 33MHz and the 486 requires only 0.1% stability from its clock source.