Just because it’s more ubiquitous doesn’t make it a better plug.
DP 2.1 is technically superior to HDMI in many ways, especially since it’s the protocol that runs through USB C and supports daisy chaining, unlike HDMI which has to be converted to and from DP to be passed through USB C.
I don’t see your argument with “it’s a better plug” either, the DisplayPort connector is available as a locking or non-locking connector, is keyed better than HDMI and in my experience is more solid.
The only reason HDMI is as common as it is is because there’s a consortium putting money into it to keep it popular because they can charge license fees from manufacturers. DP is free to use and developed by VESA, the standard can be downloaded directly from their website and added to whatever device you’re developing with no need to license. HDMI charges licensing fees PER PORT, which is why you often see GPUs with one HDMI and three DPs.
TL;DR: HDMI is only more common because a group has in its best financial interests to keep it popular, despite it playing catch-up with DP.