The main problem was that the agents weren't prepared at all for this. They simply didn't expect it. And the second thing is their own safety. Could they have done anything? Stepping out of their hiding place into the unknown? They didn't know how many shooters there were or what equipment they had. So they didn't move.

The vehicle they hide in didn't had any openings (Dutch words: het was een aanhanger die je gebruikt bij verhuizen). So they probably couldn't recognize the shooters.