Go take a look at the mortgage insurers. Their industry association is the Mortgage Insurance Companies of America, and I believe their site is micanews.com or some such.
The short answer is that PMI is primarily done on conforming mortgages, most of which get sold to Fannie and Freddie. During the boom times, the mortgage insurers were squeezed because piggybacks were popular and cheap, although they all have enough exposure from those years to get bloodied. But the primary beneficiary of PMI is Fannie and Freddie, and they are getting paid by the mortgage insurers.
"All animals are equal, but some animals are more equal than others."
- George Orwell