There is no guarantee that Apple builds a Skylake version of the RMB.
For all we know, sales of the new MacBook may be exceeding expectations as-is, and if Apple decides to keep the Air around as an active part of its notebook lineup then giving that line Skylake exclusively adds differentiation. Remember, where the RMB's place is in the assortment:
Pro: Powerful workhorse
Air: Powerful ultrabook
RMB: Sleek portable
The closer the RMB gets to the Air, the less important the Air becomes. Giving the RMB a Skylake processor is counter to their marketing strategy, same as what they did with the Retina display. Apple needs to give salespeople a very clear line of differentiation, a story to tell that makes the RMB and the Air dissimilar enough as to justify one over the other. If both lines have Retina displays and Skylake processors, then the only story for a salesperson to pitch in the stores is screen size and form factor and that's not enough.
BJ