Almost certainly because the SoC they are using has native HDMI onboard, and no native DP

> displayport [...] royalty-free.

If I remember correctly (and that's a big if), DisplayPort started royalty-free but then they introduced royalties. From my blurry memory: I checked once many years ago and it was free, but then I checked again a couple of years later and it wasn't exactly free any more. Did it change again? Or am I completely wrong from the start? If someone remembers the history better than I do...

> more common

More common in monitors perhaps, but I don't think I've ever seen a TV with a DP input. Since AFAIK the target market of the RPi is "plug it into a TV, plug a cheap USB keyboard and USB mouse, and you have a working computer", having HDMI output is a requirement.

Exactly, you can solve the dual-display problem via daisy-chaining a couple of DisplayPort monitors instead. Using Micro HDMI was a bad decision in my opinion.