Maths A-level is deliberately difficult. It isn’t meant to be for everyone. If you don’t get a grade 8 or 9 at GCSE, you are not in the top 12 percent of the cohort.
Compared to other subjects, maths is entirely cumulative. Every concept at A-level builds upon concepts learnt earlier. If you have gaps in your knowledge (which you will have if you got a grade 7 or below) you will find A-level concepts very difficult or impossible to grasp making lessons inaccessible. You will not be able to progress until you fill those gaps and unless you are very dedicated you will not have time to both fill gaps and stay on top of new content. This can lead to gaps compounding over time, leading to grades E and U at A-level.
Some other subjects are partially cumulative, but none to the same extent as maths.
It is also true that maths can only partially be taught. We all have a ceiling in maths: the level beyond which we cannot progress even with the best teaching. For some people, maths A-level is beyond their ceiling.
I really don’t understand your third paragraph. More people get high grades at maths A-level than any other subject. It is the most popular A-level by far. A higher proportion of the country gets good maths A-level results than any other subject.
However, because maths is prestigious, most people who are able at maths take it. So someone who is not very able (eg a 6 at GCSE) is going to be competing against the tens of thousands of much more able students at Alevel for the top grades. Other subjects don’t have quite the same profile.
I disagree that everyone should do maths until they leave school. And I am a maths teacher and a Tory voter.