I don't know why Apple hasn't enabled extensions in Mail for iOS 8. If time constraints are to blame, hopefully Dispatch and CloudMagic are proving that the ability to act on messages and integrating with storage providers is a much needed step for processing email on iOS devices, and Apple should take note.
I love the elegant interface of Mail and its new gestures for triaging messages, but the same limitations of six years ago still largely apply. For Mail to be truly desktop-class, Apple needs to build support for the latest iOS extensibility features, and I'm looking forward to updates next year.
Totally right. My number one wish list issue for iOS 8.