I also wonder if it could be related to the kinds of lawsuits that a church could be up against. I just recently blogged on the whole problem of sexual abuse cases in Protestant churches. Of course, there have also been high profile cases regarding sexual abuse in the Catholic church too. These kind of lawsuits can be very expensive for the church -- and for the insurer that provides liability insurance to the church.
However, I haven't found a comprehensive answer to this issue. Because the underwriting rules for insurers are "company confidential", no insurer will discuss them in detail.