Why is it so hard to recruit good maths and science teachers?

Australasian Science:

There are well-recognised problems with student participation and achievement in maths and science. Widespread shortages of suitable secondary maths and science teachers and low levels of students studying these subjects at secondary and tertiary levels are not just problems faced by Australia, but have become international issues.

A number of interrelated factors that form a self-perpetuating cycle contribute to this situation.