Historically, Apple has always updated MBPs when a new generation processor was available, unless an even newer generation was right around the corner (<6 months out), as in the case of Broadwell .
The arguments for Mac Pro vs MacBook Pro are not valid because they have not had analogous update cadences . MPs have had much slower cadences than MBPs.
The reason MBPs have not been updated is that the Skylake processors corresponding to them are not out . Theoretically, the 13" could be released since the 28W part with Iris 550 graphics is out . However, there is a planned 28W part with Iris Pro 580 as well - something Broadwell never had . If Apple plans to use that for the 13" then it follows that neither the 13" nor 15" have their respective Skylake parts yet .
It's commonly argued that 'sky lake is already available' . But no, Apple has for the last 5 generations always used only the highest end iGPU based processor in its MBPs - 28W for the 13" and 45W for the 15" . Neither of those with Iris Pro 580 are out. They are due for Q1 2016, possibly at CES.