I guess that's understandable since NICAM was a European standard, and it wasn't universally adopted across Europe for that matter.
There were probably many factors at play. Japanese manufacturers seemed to wait until NICAM was officially launched before making sets with NICAM decoders. Bear in mind that many Japanese sets from the 1980s to the early 2000s were manufactured in the UK so they were hardly dealing with NICAM from thousands of miles away.
Yep - I'm aware that Japanese-brand TVs were manufactured in the UK (Sony still have a manufacturing operation in Wales, now well known for manufacturing Raspberry Pi computers)
What isn't so clear is whether the sets made in the UK were designed in the UK, rather than just being built here, and whether the country-specific features were dictated by UK teams, or decided by Europe-wide teams, or by teams in Japan. Manufacturing isn't the same as design.
There could be parallels with Teletext in the US where Japanese manufacturers did not include Teletext decoders as standard apart from on some high end sets also sold in Canada.
I think that was a different situation as North America trialled at least three different flavours of teletext - and there wasn't a clear market leader. At least with NICAM there was only ever one UK stereo standard to consider- you just had to decide whether it was definitely going to be a public success (not whether it would be transmitted or not)
The 525 version of WST (which was used by TBS and supported by some sets in the US) hadn't hit anything like critical mass, and none of the major groups had really bought into it - unlike NICAM which prior to official service-start, was clearly going to be universal in the UK. A 525 version of the French non-WST system, Antiope, was trialled by CBS/NBC and PBS in LA, and there was a third system called NABTS (which I believe was the only standard considered and used in Canada).
With no clear leader in the US I can see why Japanese manufacturers adopted a wait and see approach.
Ironically analogue stereo was what effectively killed US teletext, as US TV set manufacturers next generation sets which added stereo support, removed teletext support.