I suspect you'll find that Apple are waiting for x-point to ship, and you'll see MBP and MB re-designs to incorporate it in the near future.
To make the most of x-point, the board will need a major re-design and doing a re-design for Skylake right now is a waste of time when they're just going to need to do it again in a few months.
10-20% CPU speed increase is the difference between a 1.1 and 1.2 GHz processor. You will not notice that in general use. The 40% GPU performance boost? Sure. But I do not think that is enough to rush out a new model simply to claim "SKYLAKE!!1" when there is other stuff coming very soon.
Don't get me wrong, I think as an end user it would be nice to see a Skylake MB. But from an Apple logistics and tooling perspective, its not worth it until x-point is here.
X-Point is going to be a revolution. It is going to mean INSTANT sleep/wake from sleep. FAR faster IO than SSD. The ability for portables to deal with massive data sets is going to grow immensely.
x-point would be outstanding. I will be really impressed and willing to wait if they do it "in the near future" however, ee times says not until Jan 2017. http://www.eetimes.com/document.asp?doc_id=1328682