What is the difference / reason for SOS to get their own insurance?

The OUC program is intended for individual/local clubs and works well for those, especially if made mandatory for all members. SOS is a provincial wide organization with many chapters as well as a board of directors overseeing the corporation and hundreds of members and as such is not the best fit for the program as outlined below.
As of January 2015, the only appreciable difference in coverage is that the OUC has increased their coverage to 5 million per occurrence. From an administrative perspective; previously insurance coverage was optional with low enrolment and so not all participants were covered. Now all members are covered (see above).
Previously, events/activities had to be registered with OUC or they were not covered. Due to the nature / size of our organization, on most occasions, events/activities were not registered and therefor not covered. Now, all corporate and chapter activities are covered at all times (see above).
Previously our web based membership system was extremely complicated and added many hours of work for the webmaster and membership coordinator. In addition, the cost of customizations for our website were significant and would continue to stay high as we upgrade / improve our website. Now, SOS members are automatically all covered and our volunteer staff do not have these complex programs to administer.
Previously the cost burden was passed on to the member, which added another $30 per year to an average SOS membership (would have increased to $40 per year in 2015). Now, liability insurance is part of the corporate budget and is automatic for all members.