It's basically what I stated earlier. Congress are wimps who always want that excuse to get out of how they vote. Spineless idiots will say things like, "He tricked me!" rather than own up to their responsibility. People believe them, because they do not understand that they have direct access to intel, they had no reason to rely on the President for intel leading them into voting us to war, but not "really" voting us to war.
Their fear of reprisal, is so overwhelming that they'll use any excuse against taking the power that is theirs over wars.