Well, he won’t, unless things start to go wrong for him. There’s no need for him to do so, given that he is not going to be convicted by the Senate. He could only screw things up.
And of course, he repeatedly said he was thinking of chatting with special prosecutor Robert Mueller, but he never went ahead with that, wisely.
But President Trump is not one to back away from a challenge, so when Nancy Pelosi called on him to testify, he told her to be careful what you wish for.