Because mortgage insurance has been and remains drastically underpriced.  Look at the instant death of Fannie and Freddie in the US when their bubble popped.  The amount of money they had collected in premiums was a drop in the bucket compared to the payouts they had to make, and they went bankrupt instantly.  We are slowly raising the premiums in the right direction, but of course that does squat to fix the problem of $700 billion plus in guarantees that we have already made, with $12 or $15 billion in capital (or whatever the number now is...).