Other mistake: During the helicopter chase at the end a pilot can be seen in Airwolf's left seat although Hawke is sitting on the right side and Santini in the back.
Continuity mistake: For most of the flight shots, Mace is shown in the right seat. Part way through the chase, he's shown in the left seat, and everything is reversed.
Answer: I don't think it's ever explained why Mac refused to tell Hawke where St. John is.