I would like to add a third reason: the harm done to enthusiastic maths teachers by the constant fight against both the apathy and hostility in the classroom.
It is terribly demoralising to spend your entire day surrounded by people who don't want to learn any mathematics, who know that they don't need to know any of the stuff you are trying to teach them, and require that you continually justify your position - "what use is this going to be in later life?" et.
In further education the position is made worse by the drivel that is Application of Number, a syllabus called a key skill which is key to nothing. I spend hours every week faced by students who know full well that they do not need to know how to construct a bar-chart or rearrange a formula to get on in catering, window display, art and design or carpentry.
I love mathematics, but I am heartily tired of trying to teach absurd and irrelevant syllabuses dreamt up by civil servants to unwilling, angry students for no good reason. If maths was an option then perhaps I would have classes who have some interest in the subject.
24 Rowan Drive