I don't have the real history of industry type explanation, which would probably involve the personalities and business leanings of people working at OEM hub/headset/BB/pedal manufacturers over the decades and so is probably lost in time, but the simple answer is that large-scale technical choices in the bike industry get made based on what's simplest to implement at the manufacturing level, what's cheapest, what works from a patenting/licensing legalities standpoint, and what solves actual problems for those with decision-making power and/or increases their profits. Confusingly, there actually is a pretty strong "if it ain't broke, don't fix it" attitude, which one wouldn't necessarily guess from the recent standards explosion, but if you dig a little deeper into the rationale of the relevant actors behind each new standard you'll see that they're at least attempting to meet the above criteria. (Note that "increases their profits" is the big catch-all behind a lot of what's happened.)
There are a lot of places on modern bikes the world over where this dynamic of split metric and imperial units exists, or exists in a veiled way with a metric dimension based on an imperial size rounded off to the nearest tenth of a millimeter.
- Grips are almost all 22.2mmm (7/8" = 22.225mm)
- ISO threaded bottom brackets (1.375" X 24tpi)
- The classic frame tube diameters are all different combinations of
25.4mm (1"), 28.6 (1-1/8" = 28.575mm), 31.8mm (1-1/4" = 31.75mm), and 34.9mm (1-3/8" = 34.925mm). Manufacturers have largely moved away from simple round tubes but we
still see 28.6, 31.8, and 34.9 as the clamp-on front derailer
- Handlebar clamps for non-oversize flat bar bikes are 25.4mm (1"). The
industry did standardize a metric-based similar size (26.0mm) for
upper end road bars, but then when oversize came out everything
became 31.8mm (1-1/4"). The new 35mm standard somewhat reverses the
trend yet again.
- BMX/youth bikes usually have 22.2mm (7/8") or 25.4 (1") seatposts
- All modern pedal threads have imperial sizes (9/16" x 20tpi or 1/2" x
20tpi), even though all their wrench flats are 15mm, 8mm, or 6mm.
- Axle threads for many hubs are either 3/8" x 26tpi or 3/8" x 24tpi,
namely coaster hubs, internally geared hubs, some low-end derailer
rear hubs, many low-end front hubs, BMX fronts (even if their rears
are 14mm), etc.
And there are more examples yet. Another side to this however is that in industry the imperial system is really only the metric system in disguise, since in 1930 and 1933 the UK and US respectively began using 25.4mm as the reference dimension that defines one inch. That's why all the the fractional conversions are relatively clean numbers. But it means that the high-level work of keeping track of what defines either a millimeter or an inch in terms of physical and theoretical references is the same for both systems, and is done by the same organizations.