This was either a bodge job to create a short-term replacement for the Mac Amateur, or an even stranger question as to why the Mac Amateur was being ignored.
While we applaud Apple for finally putting decent processors into their desktop computers, they still ignore function over beauty. iMacs (IMHO) should all have Xeon 1200 (E3) or 1600 (E5) processors in them - they're not laptops, might as well put a bit of horsepower in them.
If you've looked at the technical specs of the iMac Pro you'll notice that it will take up to 128 GB of RAM - Woo Hoo!! But looking at the photos, and where the RAM is situated - Apple expects you to buy all 128 GB of RAM at original purchase. Gone are the days of the nicely situated panel at the bottom that could be removed to access / and or replace the RAM.
Maybe the discussion went something like this:
Apple has at least acknowledged the need for better networking and included the ability to connect to 10 Gb Ethernet. But again, beauty over function, they include a copper 10 Gb connector. In a day where most 10 Gb connections are SFP+, Apple is stuck on copper only by including 10GBase-T as their connector of choice.
An SFP+ connector takes up a lot more space than a simple RJ-45, so this probably had a lot to do with the reason to go with twisted pair. The downside is that owners will be limited to copper. If Apple were to utilize SFP+, users would be provided with much more flexibility - most notably, fibre. SFP+ connectors are available for either copper or fibre (multi-mode & single-mode) interconnects.