IMO, because quad core laptop processor advancements in almost the past half-decade have been pretty marginal compared to the half decade before that. In 2015, Apple got a lot of grief for not updating the CPU in the 2015 15-inch MacBook Pro over the 2014...well, now we see why they didn't bother changing it.
From a pure processing standpoint, the 2016 doesn't really seem to make advancements over the 2015. Passmark shows a certain small degree of favorability to the older 2014/2015 CPUs, but some of this would be offset by the improvements in heat dissipation, and some of it could be margin of error.
How much does thermal throttling harm the 2014/2015? Well, I have a 2014 with a 2.5 GHZ i7 and the effect is a lot less than I thought. The computer can hold about 3.5 GHz continuously. But, that also means holding nearly 200F temps consistently, along with fans running at 6200 RPM consistently. With the much more capable (and reportedly hotter) GPU in the 2015, the effect could be different. With those who have the current 2016, the computers seem to hold a similarly high speed at lower temps and lower fan speeds. Even if the effect on performance is limited, less heat can mean less wear, and fans at 4000 RPM are a lot less annoying than 6200 RPM, so there are certainly some advantages with the newer design that a benchmark would not reflect.
However, at the same time, processor efficiency, (especially) the iGPU, the dGPU, and SSDs have made significant advancements...and Apple (among other Makers) has certainly made effort to integrate these advancements into their new products since they can't magically make the God of Intel poop out a faster piece of silicon.
On a positive note, AMD is about to light a firecracker under the butts of Intel execs, and hopefully will again be able to challenge Intel's long-standing status as industry leader. Hopefully, that will mean more rapid CPU advancements & falling CPU prices (at least until the limits of silicon are exhausted, and then we are forced to move to something else.) It could also mean a MacBook Pro with an AMD CPU and a NVIDIA GPU, which I think would be hilarious.