The reason is that moisture may condense between the vapor barrier and the roof. Before any vapor barrier can be used you must perform what they call “dew point calculations” (See http://en.wikipedia.org/wiki/Dew_point) As you can see per the Wikipedia article, doing the calculations is not something done lightly.

If condensation is the issue – ventilate and insulate. Only place a vapor barrier after consulting an engineer who can perform the calculations.