I think it's down to them being too late to the market and coming out with a "Me too" range of bodies as for me there doesn't seem to be any real appeal except for those with a bag full of Nikon lenses and of course those who only see the badge and simply must have a Nikon and those two categories wont sustain them.
The appeals I see with the Z system:
- most of the S lenses are simply outstanding and include at least one piece of glass nobody else has (the Noct). Nobody else does super high quality f1.8 glass accross the various focal lengths
- the viewfinder is more pleasant that the higher res a7R IV I have started to use
- the size is compact but the ergonomics of the body are great
- IBIS works great
- colors are great and so is the AWB
- it's fully waterproofed
- you can adapt any lens on it, including the Sony E lenses so it's the closest thing to a universal lens carrier
- there is zero sensor dirt issue (that can't be said of the Sony bodies)
- video works great, in particular video AF
The only thing that's clearly not close to best in class is the AF on moving subjects, but it's still completely usable. I got a high rate of keepers with the Z6 and the 200mm f2.0.
So all it would take to Nikon to have the best range of mirrorless bodies is improved AF, and the good news is that they have been the best in AF in the DSLR era, so they already have the algos... and a second memory slot.
I am not sure that a body that close to being the best can really be called me too.
Cheers,
Bernard