Why is the iphone 6s battery inferior to the 7? https://www.cnet.com/reviews/apple-iphone-6s-review/

CNET 2018 indicates the 6s has: 


  Inferior battery life, camera performance and processor speed compared
  to newer iPhones.


What exactly is the battery life inferior? Possible reasons comprise:


fewer mA-hours battery?
CPU voltage swing greater? (greater power consumption?
Display power efficiency?
I'm not entirely convinced the author of the article you referenced did his homework! I've heard that battery improvements from iPhone 6 to iPhone 7 have been incremental. That jibes with battery technology progress in general (i.e. slow progress), and a quick search found this article that seems to back up the "limited improvement" opinion: 


  "... the battery capacity has changed little (iPhone 6 - 1810mAh, iPhone 7 - 1960mAh) and you’ll still struggle to get a full day out of it... 


Those figures are repeated in this article - a compilation of battery capacities for all iPhone models from 2007 to 2017.  One interesting thing to note from the table is that "size matters" - for batteries at least. Notice that the jumbo phablets have significantly more battery capacity, but that's down to the batteries being larger. And of course a phablet will demand more power than a 'normal' phone due to the larger display. 

For all these reasons, I'd tend to discount the claim that battery life is "inferior" for the 6S. I'm not saying that there's no improvement; I'm saying that the improvements are "evolutionary" instead of "revolutionary". "Inferior" suggests poor quality and/or significant differences, and the data simply do not bear that out. I think Apple will do what they have to do to remain competitive; that includes battery technology.