Maybe because he was expressing his personal opinion? Should he talk about that in third person?
The problem is that Trump's business success, beautiful family and incredible energy levels make a certain type of personality feel insecure. They attack Trump as a way of (in their tiny minds) narrowing the achievement gap. Due to the self centeredness of their worldview the 'attack' is always about their feelings, not what's best for the country, or what their own achievements might be. When you're short on accomplishment you need to find some other way to mark your territory.