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One of the main reasons 6.3V was chosen as a filament voltage was car radios. Back before they figured out how to convert utility voltage to something tubes used 1.5V, 2.5V and 5V signal tubes were common in battery powered home radios. Car batteries were standardized at 6.3 volts from the beginning until the mid 1950s when 12V became the norm. Car radios started to become possible and popular around 1930 and tubes started getting standard RMA part numbers around then so the industry standardized around tubes that would be good in car radios. The lower heater voltages persisted in battery powered stand alone portable radios like Zenith transoceanic. In the 30s there were also higher voltage heaters developed for cheaper radios to meet the demands of great depression America...1,2,3,4,5 and 6 tube heater series strings would divide the full 117VAC from the wall ...the 5-Tube AA5 (all American 5) became the standard of cheap enough and good enough for nearly all places and sold best. Single 117V tubes were common in one tube wonder phonographs, and were sometimes used in battery/AC portable radios... Sometimes as just a rectifier, but some 117V tubes had both a rectifier and power penthode that would be used in lieu of the battery output tube for more audio power when running from AC.
Not sure why rectifiers favored 5V (they may have adopted that as a standard before car radios), but there were exceptions like the 6X5 and oddballs like the late 50s introduced 3DG4.
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