Why 6.3 volts?
 

Time to ponder a new question. Why did 6.3 volts seem to become the defacto standard for fillament voltage for so long ? Early radio tubes that needed 2.5 volts were run at 1.5 to 2.5 and we'll leave out series string for now. Was it a transformer thing like only takes one layer? Power? Na, power is the same we just jugle the volts/ amps.
Why 6.3?

Seems like a coincidence that a car battery of the time was 6.3VDC. So maybe the choice of 6.3V was to enable the use of lead acid batteries for powering the filaments?

Dave

Zenith is correct. It's due to automobile radios. Or three cells in 'farm' radios.