Mostly market strategy, with some practical design benefits.

Lenses with narrower maximum apertures are usually smaller, lighter, and less expensive to make and buy. That fits one of mirrorless cameras’ big selling points: a more compact system. Launching a new mount with only large, expensive lenses would make the system less appealing to many buyers.

They also tend to sell in much higher volume. Most photographers don’t need the last stop or two of aperture enough to justify the much higher cost, size, and weight of faster lenses. So releasing the more affordable lenses first helps Nikon get more users into the system quickly and gives most customers what they need sooner.

There’s also a transition factor: photographers who buy premium fast lenses often already own high-end DSLR lenses they can adapt, so they’re less urgent buyers for native mirrorless replacements. Meanwhile, more mainstream buyers are more likely to purchase the smaller, cheaper native lenses right away.

So the order is likely driven mainly by demand, price, size, and system adoption—not because faster lenses are unimportant.