1) Some IT organizations require an outside support organization for
all components in their infrastructures, by policy. It is perceived as liability coverage in the event of severe problems. The jargon for this is "having a throat to choke."
To me, this has nothing to do with infrastructure quality, or the mitigation of risk to business functions. Instead, this policy provides management with the ability to assign fault outside the IT organization, which mitigates risk to management careers.
2) Compliance to government regulation or industry standards, or infrastructure in support of a particular application may require specific and limited solutions, eliminating many options of free choice.