Reading https://danielpocock.com/en/mailing-lists-vs-discourse-forums-open-source-community-or-commodity/
I believe that the main problem with mailing-lists is that most email clients sucks and people are just trying not to use them.
And seeing how parodicaly bad Outlook is, you start to wonder if it is done on purpose to promote proprietary systems and kill the last open/universal communication network.