Nikon will come out with 6 lenses in 2019, and 7 lenses in both 2020 and 2021, so far. In other words, Nikon will have 23 Z lenses by the end of 2021. Eleven of the 23 lenses are "To be Announced." Not bad!
IMHO, that is the main reason, not the only one, Nikon developed a Mirrorless camera with a flange of ~10mm and not ~16mm, which is the flange of their DSLR's. Therefore, it necessitates a new mount (Z-Mount) and a slew of new Z-Mount lenses and a ZTF (Z-Mount to F-Mount) adapter. Very good for Business!