The biggest problem is so much of music notation's meaning is defined by it's graphical information, not just the content. This is where it differs significantly from code or even text typesetting. At some point, you have to get into the nitty gritty of where to place symbols on the page. Lillypond gets you part of the way, but the last part is really hard to do and most users are more comfortable with a WYSIWYG setup.