I think the underlying problem is that the UK education system is based on specialision. Pupils chose their GCSEs and A-Levels with the mindset that they need to pick the exact subjects which will help them go to uni and/or get the best job they can, and as we're being told we have to specialise and only pick a handful, no one wants to "waste time" on creative subjects which they could do outside of school (which they don't end up doing because GCSEs and A-Levels take up so much time). Maths and English are obviously more well established, and I think that's going to stay the case because they fit so well in this specialisation system - they're seen as teaching widely-applicable skills (problem solving, writing, analysis etc) that could be useful even if you decide to change what industry you're going into.
In order to even be seriously considered, DT subjects needed to be more academic (also IT, which is slowly being phased out in favour of Computer Science anyway), because otherwise (as someone else mentioned) they just remain subjects that aren't taken seriously. However, with this change they're less appealing to students who are less maths/science-academic and better suited to creative subjects, and STILL won't be appealing to more conventionally academic students who would want to go into tech and engineering because they're safer taking Maths and sciences courses to get into those sectors anyway.
I think the same applies to all creative subjects. I would love to see more people in Music and Drama at my school, but very few people who actually have a talent for this kind of thing actually want to take them. There's too much pressure to pick academic subjects, and the thing that most people enjoy about Music, Art and Drama is the creative side, so when music theory and artist research are suddenly injected into the course after a year of practical lessons, students (at least in my school) just drop them anyway. As long as we have this system there's not much point in making creative subjects compolsury. It would be great if schools' performing arts departments got more funding to run concerts and productions so students could have experience in creative arts on the side of Maths, English and Science without having to take an exam in it (à la US high schools on TV), but not many people past Year 10 who don't take a Music or Drama course would participate because it's not going to help them get an A.