berlinonaut

It is simpler than that. The Brompton's rear end has just 112mm OLD. This means that most hubs simply won't fit. Being a child of the 70ies Brompton used 3-speed hubs from the very beginning, later offering 5-speed hubs as well. And being British those came from S/A. When S/A went bust in 2000 they were in trouble. They managed to fit the Sachs 3-speed hub to the Brompton but their 5-speed (called pentasport) was to wide and could not made a fit. Not a big loss in my opinion as this hub is a nightmare of it's own. Lacking alternatives they developed the half-step 6-speed that they still use today - it got invented in 2002, at this time based on a Sachs Torpedo/Spectro hub. Until it was ready Brompton offered Schlumpf drives to enhange the limited spread of the 3-speed hub. Heavy and expensive, but mechanically very sound.
Sachs had a halfstep-kit for it's hubs on the market already back in the 1980ies (I had one of those back then), therefor the hubside was not a big issui. In 2005 they went back to S/A for the 3-speed, the 6-speed remained Sachs (now being SRAM) until the invention of the BWR in 2009 that finally offered gear-spreading equivalent to a modern 8-speed hub.

While Shimano 7-speeds have been fitted to the Brompton as early as the late 90ies this has always been a custom build as it needed spreading the rear frame. Same goes for the Nexus (which indeed is better than the 7-speed and not that much more expensive). Juliane Neuss from Germany offers the fitting of the Nexus commercially and has done several hundred conversions as far as I know. Works reliably but as said before: Fitting is a bit tricky and you have to know what you are doing.

Brompton does not like Shimano (the only japanese part they fit is the Shimano hub dynamo and this is not a very good product - they went for it due to lack of alternatives back in 2010 as a cheaper alternative to the SON). The 1 kg weight penalty through the 8-speed as well as the necessary wider rear frame (spread to ~124mm necessary) are both prohibitive for a factory option from Brompton. The more for the Rohloff which is wider and cannot made slimmer. Over the last years a range of options showed up for those who really want a Rohloff in their Brompton: First was Steve Parry, then there is Ben Cooper/Kinetics with his custom rear frames and lately the guys from Russia (Vostok) made a 135mm rear-traingele out of titanium for the Brompton.

I know people that own a Rohloff Brompton and they love it but - despite living in hilly areas - say it is a bit overdone and not really necessary on a Brompton.

One could also go for the Alfine 11 but again it is a hefty penalty in weight pus massive spreading of the rear frame. Plus it is a Nexus 8 on stereoids with a very short first gear and to additional ones at the top end and unfortunately very prone to exact shifting cable friction and tension - not optimal for a folder like the Brompton. The e-Version exists but is even more expensive.

So in the end it is weight, compatibility, reliability variability and price that lets Brompton step back. I'd assume they possibly tested every single possibility over the years. They could however easily use the S/A SRF5w - basically a more modern version of the Sprinter that they used to fit in the 90ies, basically as robust as the 3-speeds it is based on, and would get rid of the dual-shifter setup by doing so. But as they have their six-speed they do not seem to see the need for that.