Really, the iPhone 7 was just the 6SS. The 'S' revision was spec and camera bumps, but no major case changes and the 7 made no case differences over the 6S.

But probably a 6SS or whatever wouldn't have sold as well, so they shipped a 7, and now the 8 actually does change the case significantly (glass back) and so thats not a 'S' revision.

No product cycle will play out perfectly in a versioning scheme, and your only surprised about the missing 'S' because they have had a naming scheme that has been surprisingly consistent for this long!

Next year will be the iPhone A...