Could it be that the Nike edition would cannibalize sales of regular aluminium models. If you go with a Nike version, you get extra watch face options for the same price. Who would bother with the standard aluminium versions on launch day? They probably just want to make sure non-nike inventory actually get's purchased.
It begs the question, why not just make the aluminium version and the Nike version one and the same?
Because then Apple has to give Nike a larger cut of the profits.