People who write the ISO standards are required to transfer copyright to ISO, and lose control over their work. ISO says the paywall is to pay for development, but the authors don't get a cut of the sales.
https://twitter.com/isostandards/status/1367138676162105344
I don't know how these things usually get done, but the JPEG XL standard has been written for free by volunteers, and ISO paywalled it. The authors would like to change that: https://twitter.com/jonsneyers/status/1415696597585993732