I also think a big reason is Apple wants to give a cheaper entry point for those people who probably could (performance-wise) get away with an MBA, but also "want the Pro". Whether it's as a "lifestyle / professional accessory", or to some small degree a "status symbol", Apple has always had some part of that as a part of its brand. They've historically been a more premium, luxury product brand, although that has waned a bit as Apple has grown to the mega behemoth it is today.
I could easily see someone who could otherwise be fine performance-wise with an MBA, but thinks "hey, for just a few hundred $ more, I can get the ProMotion screen, an HDMI/SD card port, AND get the slight coolness of having the Pro? I'll do it."
Apple did the same thing with the previous generation MBPs that came out in 2016 w/ Touch Bar. After a year of the new model, they came out in 2017 with a cheaper entry-level version that had no Touch Bar and only 2 TB3 ports. It was the same thing - a cheaper entry level way to get "the Pro" without the full fat.