This is, of course, not a problem which is limited to a subject like A-level music. There are many subjects which are not regarded as mainstream, including modern foreign languages, where very small class size makes it a difficult subject to offer.
Perhaps the universities are also partly responsible for this. With the rising cost of university education, most students want to study a course which will lead to employment. A-level music is not always regarded as an academic option although it is often required to pursue music at tertiary level. Of course the problem with studying tertiary level music is it does not lead to a wide range of employment options.
I think students feel it is safer to study a subject at A-level (or its equivalent) which is more understood by admissions officers. Getting into a business or science course with one of your three options being A-level music might be possible but a lot of students may be concerned about how music will be perceived by those admissions officers.
If universities could place positive encouragement, in some way, on less common or popular subjects, students might feel more confident in including them in their A-level choices. But at present, music is often seen as an extra-curricular choice, rather than part of the mainstream curriculum.
Genius is one percent inspiration and ninety-nine percent perspiration.