Professionalism: Without a licensed professional conducting the investigation, chances are extremely high that the findings will be inadmissible in court. 

Credibility:  An internal acquaintanceship could be perceived as a bias in regards to the findings of the investigations at trial, up to and including the dismissal of collected evidence.

Expense:  In-house IT staff being removed from their regular duties raises the cost of regular salary expenses, plus the value of lost time due to the investigation.

Experience:  Professional investigators are constantly trained and educated in new techniques and other factors that take considerable time to gain proficiency.

Knowledge:  Adherence to legal issues in order to maintain acceptability in a court of law is a known issue to professional investigators.  In-house staff may unknowingly and easily overstep legal boundaries and destroy the entire case.

Loyalty:  Internal personnel may have contacts that can conflict with an impartial review of the facts, particularly if the investigation reveals connections to a professional/personal acquaintance.

Man Hours:  Forensic investigations can often take hundreds of hours of intense effort.  The cost of same in terms of interruption of business would be difficult to quantify.

Objectivity:  Internal staff may concentrate on looking for "evidence" that coincides with what their employer is seeking to discover, possibly "finding" material that does not actually exist.

Security:  It is literally impossible to keep a forensic investigation conducted "in-house" confidential, potentially raising privacy issues among employees.