Everything I know about HDR is that it's a pain in the ass to work with, it doesn't translate back to looking great on a non-HDR monitor, and it looks great only when carefully done to look great (or automated to look great, which isn't necessarily "looking great" for the content so much as just a general, "Oh, that looks great" because the algo cannot be 100% natural to the content but it can pick up stuff people like at a quick glance like real deep, crushed blacks.) Games are great for HDR because they actively render right to the display (and yet that still gets screwed up,) but once you sever that communication between the hardware and the display, you have locked recorded footage and are locked into whatever constraints come with what/how you captured. Also, I believe it's lossy, even though the range is higher it transforms the content rather than having an additive channel, so you would think HDR would be just SDR plus a high-dynamics range and would be like resolution where you could scale back down to the old way easily, but I don't believe that is so comfortable a case.
So, for publishing a trailer to the internet which will mostly be watched on phones or work laptops, it makes sense why HDR just isn't happening with game trailers. (I believe you would have to master it start-to-finish in HDR to really get it right, not just apply an HDR pass of the completed footage. Also, I'm not sure how cutting actually works in HDR, if there are times where cutting between different dynamic ranges is something to be careful of, similar to cutting on depth if you have a 3D project?)
A game clip or even a livestream, that makes more sense because you can narrowcast to a select customer base looking for HDR footage. A trailer is too important to hit perfectly every time a new eyeball falls upon it, so you would want none of the HDR fibbergibbity and standards-bickering to impede the first impressions of your big title.
(*Please feel free to correct any/all the technical fuck-ups I made in the post above. I know people who grumble about having to work with HDR, but I have little personal experience with it...)