Because teachers and nurses have no power, that's why.
Teachers have no power, apart from the ability to form the entire country's mental worldview. And I think it's pretty clear that having 90% of the primary teaching population consisting of women is doing considerable damage to boys' academic achievement.
It matters that power is not concentrated in the hands of a few middle-aged white men because on the whole, the people in power put through policies that benefit them and people like them. The current lot in power have no understanding of the lives of ordinary women - and that shows in their policies (and their poll ratings).
What makes someone with a PPE degree from Oxford and a vagina better than one who has an identical background but a penis? Gender quotas won't solve Britain's problem which is half of the HoC is made up of people who, a hundred years ago, would be called 'gentry'. That's the case throughout all the three parties. Do you think that because these privately-educated, upper-middle-class people don't have schlongs that they understand what it is to raise two kids on one average salary?
And no-one was accusing you of being sexist.
Covertly, you were. You wrote 'unless you think there aren't enough women capable of being ministers'. So if I say 'yes, there are not enough women' you can accuse me of sexism. If I say 'no, there are plenty of women' you can say 'why not use quotas to push them ahead?'
I'll tell you why I oppose quotas for politics and for business. There aren't enough good quality women candidates. There are two good reasons for that, and only one of them is fixable with quotas.
The first reason is that a 'culture' forms which excludes people who 'aren't like us'. This happens in every field. Take journalism. Look at the number of wealthy Islingtonites who go in to the media profession. Look at the BBC. Tell me why it is that there isn't a single Marxist MP or councillor but the BBC Comedy controller manages to find seven of them to go on Radio 4 and tell jokes about Thatcher.
It's "clubbability". People hire people like them because people like people like them.
Quotas would force this to change.
The second reason is that women make the decision to cut back on their careers to have children. It's biological, it's genetic and it's just how they are. Women outearn men until their early 30s. The reason for that? Career breaks to raise a family. Politics is a hard business. Being an MP is a seven-day-a-week job. Have you seen an MPs diary? Unless the woman has a househusband or staff, being an MP and mother is almost impossible.
You can't change that situation with quotas.