Music notation is excruciatingly complex, especially when it comes to edge cases and more esoteric notations, and the artistic intents of the engravers. It's not just about "copy these notes from here to there" there's actually a ton of other aesthetic information that should be captured as well, that both makes the score more readable, and informs the performers about how to play the music.
Asking to cut & paste sections of music is basically like asking to support cut & paste for sub-fragments of SVG objects. It's "possible" but both technically difficult to implement, and of questionable utility.
To get a feel for even some "regular" cases of complex music notation take a look at the LilyPond snippet repository here: http://lsr.di.unimi.it/LSR/Browse or even at the LilyPond documentation itself: http://lilypond.org/doc/v2.19/Documentation/notation/pitches
So it's becoming a standard.
As their new year message (https://www.w3.org/community/music-notation/2015/12/10/2015-... ) says: "we look forward ... to start more extensive work on an updated web music notation format for the future."