In my experience as a consultant, this is the case because Business Managers don't trust IT because:
1) The IT department inserts (perceived) too many non functional requirements on a project increasing cost or schedule.
1.a) (Perceived) The IT department doesn't care about the needs of the Business Manager's daily business.
2) Internally the IT department did not deliver on it's own projects within cost or schedule.
3) There's no way that an employee could be as smart as a consultant.
Having been a former IT employee and now a consultant, points 1, 1.a, & 2 are valid, point 3 is just bunk. Now being an consultant, I prefer to work with Business Managers because:
1) Business Managers have a vested interested in seeing a vendor project complete, where as IT typically does not, it's not their money or idea.
2) Business Managers will make time to meet with a vendor, where as IT typically think of vendors as hired hands, about as valuable as the lady who vaccuumes the floors every night.