Well, to be fair, I doubt it is a case that somebody decided to strip out or ignore those items just to be mean-spirited or proprietary. I think you (and others) have made great points about the difficulty of mapping MusicXML’s presume structure to the more abstract (and powerful) structure that Dorico uses. I think you also make a good point that some clever programming could map at least the most common items into a highly usable form as the XML is imported into Dorico.
But that takes time, and therefore resources that must be prioritized.
V1 was about getting a functioning program in place.
V2 was about getting the bare essential that a majority of writers needed.
V3 is about enabling large score, guitar tab and a whole bunch of really useful foundational items that couldn’t be tackled earlier.
That leaves some big areas:
More advanced playback (scrubbing mode, observance of rit/fermata, scoops & bends etc.)
Better DAW integration and/or more DAW-like control over the MIDI
Document interoperability (i.e. better MusicXML import/export)
Hand input (draw notes, articulations. dynamics, slurs, on a multi-touch screen, etc.)
OCR (probably outside the scope of Dorico proper, but hopefully some advanced technologies in the future)
… and probably many others. I’m hopeful V3 is not the last release.