Although tubes were already non-economical when I learned electronics, they were still hanging around, the books still focused a lot on tubes, and that is what the teachers knew and loved. Naturally I've forgotten just about all of it, but I'm trying to remember. Although tubes can handle 100s or 1000s of volts without difficulty, the usual plate voltage in an old radio was around 100. I think that was simply a convenient voltage because that is around the US line voltage. Radios often operated without a transformer, or only a filament transformer, so a voltage around the line voltage was convenient, minus some for the ripple filter in the power supply. Although tubes can be built to use lower voltages, there was no particular reason to do so. I believe portable radios (using batteries) used lower voltages. I'm sure tubes can be designed to use 12V as long as the output power is small.
Although operating at a low voltage is now the norm, it was a difficult problem for transistors when they were new. For one thing, all components were designed with the parameters of tubes in mind, and those for transistors were very non-standard values and therefore expensive. For another transistors were super-sensitive to over-voltage. One microsecond and they are gone.
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