Sony owns a stake in Tamron, they might not want Tamron to licence the G2 version to Nikon.
Tamron might not like to licence their newer design to Nikon, or might want to keep the option open to include the G2 in a future Z-mount line-up for themselves.
Nikon might not be able to easily adapt the linear motor design of the Tamron G2 lens to their Z protocols, so they chose the G1 version with stepper motor design.
Or what Tamron asks for the G2 version licence might push the price too close to the Z 24-70mm f2.8S Nikkor.
Or the Tamron G2 would get too close to the 24-70mm f2.8 Nikkor in performance, harming sales of their more expensive and lucrative lens.
A quick look at the manufacturer MTF charts suggest it might actually be the last reason:
Tamron G1 and G2 MTF:
Nikkor Z 24-70mm f2.8 S MTF: