Generally speaking voltage-level preference is a compromise between local suppliers of utilization equipment and utility companies that supply power. So far, there is no world-wide standard although the European community (EUR) has established uniformity amongst European countries! (An aside: the USA, and a handful of others, hasn’t even accepted SI-units yet, although it officially agreed in 1866!)
In the USA Standard Nominal System Voltages, e.g., from 120 thru 230,000 V, are established by the American National Standards Institute (ANSI), listed in National Electric Code (NEC), published by the National Fire Protection Association (NFPA), and enforced by the Authority Having Jurisdiction (AHJ.) By comparison, the voltage-levels you cited are more closely related to European practice, which, until introduction of IEC, were different depending upon the country of manufacturer!
The voltages 33kV, 11kV, 6.6kV etc, is due to the Form Factor. Where form factor for sinusoidal waveform is the ratio of RMS Value to Average Value. i.e 0.707Em/0.637Em = 1.11
where Em = Maximum Voltage. Because of this factor the voltage levels are in multiples of 11, 1.1, etc.
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