The problem is that email has been taken over by a handful of providers, all of which have "growth & engagement" as their business model and would rather not implement any open protocol at all.
IMAP remains supported for backwards-compatibility, but I'm sure its shortcomings are seen as a feature to push users towards the official clients so there is no incentive to implement another open protocol to address them.
IMAP remains supported for backwards-compatibility, but I'm sure its shortcomings are seen as a feature to push users towards the official clients so there is no incentive to implement another open protocol to address them.