People don't want email, they want a set of office products.
In Germany we had the "de-mail" which was supposed to be an official email service that's accepted by government offices as a legal means for better communication than fax machines. Now, a couple years later, most ISPs that offered this service, have given up on it and shut down the infrastructure for it.
I'd argue there's no value in providing email as a service (except for marketing and spamming), and that is the reason why everyone migrated to microsoft's O365 package because it integrates with other business processes nicely.
Maintaining your own email server is also ridiculously expensive. Not only the maintenance and unblocking overhead, but the legal compliance as well. You'll get government-issued requests for sharing customer data probably on an hourly basis.
Do you want to deal with that in order to keep it a legal business? Probably not.
There's also little room for innovation due to how email clients work, and how legacy tech has to be able to interact with each other...except maybe with (managed) GPG encryption for a secure communication pipeline, but then you need a good contact management system, which by definition will end up with basically the source of requests of the feds for the keys...and then you need to figure out how to encrypt user data with them inserting their keys at every login, so that it's actually only in encrypted form on your server.
And then you get a legal case that buries your resources for years like it happened with tutanota, which in the end forced them to send a copy of all incoming emails to the feds.
And then you just give up because email isn't worth it.