"VR/IS is probably not mature enough to be that reliable and durable. Is this the reason?" No. If that was true, they wouldn't put it in any lens used for decades. All the telephoto prime lenses come in variants with VR/IS.

VR/IS likely adds weight, size and cost. For this particular lens, Nikon decided it's not a trade-off they want to make.

I thought VR mechanism from, say, the new 70-200mm could be reused virtually without any changes for, say, a 105mm prime.

I'm not a lens designer, so this is pure speculation: I doubt that you can as easily adopt that VR/IS system. One reason being that it's prime lens vs. zoom lens and the other that it's f/1.4.

it won't necessarily be used at ƒ/1.4 all the time.

That's true. But the VR/IS system likely still has to move the same amount of glass around, no matter what the aperture is set to. If you have to push your car, opening the windows won't help much. From an engineering standpoint, you cannot make a trade-off here, because reducing the aperture merely closes the aperture blades, which gives no advantage in mass. (again, no lens designer, just assuming that moving mass around is the primary job of a VR/IS system)

There's also one thing that VR/IS definitely adds: vibration reduction/image stabilisation. Nikon does not only sell this lens. If they add VR/IS to this one it might be too much of a competition for other products in their lineup, with an added tele converter for example.

With all things considered, Nikon decided to make the lens the way they did. The premise that this decision has to line up with your own expectations and market observations does not necessarily hold.