Branding, you're never in doubt when some YouTuber is building a PC using Noctua fans. There's probably some weird psychology in the colour as well. Why would anyone want a fan in such an ugly colour, unless it's really good. The weird beige and brown is highly recognizable, and even if you already don't know about Noctua you're likely to ask questions about the fan solely based on the colour.

The black fans are really only needed by those who build show piece in those cases with glass panels, but they might equally well need white fans, which Noctua doesn't make. Personally I don't really care about the colour of my fans, you can't see them anyway.

They are enthusiast targeted brand and majority of enthusiast (heck even entry level cases) have windows in it.

Enthusiasts care about the details - airflow, cable management and of course aesthetics. Noctua doesn't. I respect their engineering/no bullshit approach, the price bump is worth it but they lost my money on multiple builds by sticking to their 'brand color'.

They're the Soylent of the fan world - everything you need but spark zero joy.

I'd be interested in seeing sales figures for same fans in their brand and other colors.