Looking at just Macbook Pro retina, which they skip a generation of Intel processor, my speculations:

* They have issue getting USB-C/Thunderbolt 3.1 right, power usage wise

* They are trying to incorporate TouchID, which requires custom chip like in iPhone, such chip takes time

* They want P3 extended sRGB for the retina screen but has issue with supplier.

* They are building a new trackpad that can support Apple Pencil, and it takes time.

* They have move majority of Mac hardware people to focus on ARM chip based Mac.

* They have a brand new overhaul design, but Johnny Ive is too busy with flagship building

* They are teaching customers that you only need to buy a new Mac every 5 years.

But there is no excuse for Apple not to make an interim upgrade with whatever new generation of Intel CPU.