They simply weren't possible. HVDC relies on semiconductors that are relatively new.
HVDC is older then semiconductors, but HVDC systems using solid-state devices are much more practical.
https://en.wikipedia.org/wiki/High-voltage_direct_current
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HVDC is older then semiconductors, but HVDC systems using solid-state devices are much more practical.
https://en.wikipedia.org/wiki/High-voltage_direct_current