It was Henry Greenly, who had a great deal of influence, precisely because he promoted standardisation to sort out an unholy mess of scales and track dimensions perpetrated by the early toy train makers. But he tended to think in terms of diningroom table modelling, more's the pity. For example, he proposed (or copied from early US attempts) that HO be
1/8" to the foot instead of 3.5mm, even though he also proposed the 7mm/ft scale for 0, which became the UK standard. That's a rounding up from the Continental scale of 1:45, or 6.76mm/ft. (O scale is still a mess, with 1:43 for the UK, 1:45 for the Continent, 1:48 for North America, and 1:50 for diecast transport models "suitable for O gauge.")AFAICT, Greenly proposed 4mm stock on 16.5 mm track because a) he thought 1mm tolerance was a precise as one could reasonably expect with hand tools; b) 4mm/ft give you 1mm equivalent to 3 scale inches, making imperial to metric conversions easy; I suspect that was also his motivation for HO at 1/8" to the foot; and c) 16.5mm track was readily available, and was "close enough".
His aim was to make decent-looking model railways accessible to the "average man", an aim in which he succeeded. I just wish he'd been more tough minded about scale and standards.
Just to stir the mud a little more: 1:45 is "true 0", ie matches the track gauge of 32mm (to within 0.12mm, which is within prototype variation in gauge). Half-0 or H0 would then be 1:90 (3.37mm/ft), a scale that was actually used by some European manufacturers before and after WWII, until 1:87 became the standard. I suspect that HO became
3.5mm/ft because of rounding up; and that became the standard because of the US market.HTH