Trump is a counter-puncher. Always has been. It’s his nature.
Most other candidates did not study him or examine his history, so they started throwing punches not expecting to get hit back.
Cruz hasn’t punched because he knows what Trump is about, and Cruz holds his fire against most anyway.
Carson is a nice guy and doesn’t want to play dirty, so no punches from him.
Santorum and Huckabee haven’t thrown punches at Trump.
Guess who Trump hasn’t attacked? Those who didn’t throw punches at him.
When the GOP elite goes after Trump, Trump goes after them. It isn’t that he is unfocused. Quite the opposite. He is very focused on winning. And if you get in his way, if you challenge him, he will knock you over.
It isn’t very strategic. But it is working so far.
good analysis
I don’t think Trump does himself much good by getting into petty dust ups with the likes of Megyn Kelly, though.
If we could only get a couple more like him in the arena at the same time (not necessarily candidates) the establishment wouldn't be able to get off the ropes for sure. As it is now, a few of Trump's hits have them seeing stars.