Why didn't Trump testify?
He didn’t testify because he would not be able to just rant and rave about anything like he does at rallies. But more importantly, by being on the record he could risk perjuring himself. Apparently, his backers believe everything he says but in reality he lies and exaggerates too much that he’ll open the door to so many issues that he’ll figuratively hang himself.
It’s the same reason why his lawyers didn’t invoke the “advice of counsel” as a defense. Even though, later they tried to get some of that defense but not the entire spectrum of that defense. If they did use it, then they open the door to what was said during attorney-client privilege as that privilege wouldn’t be applicable. The truth will come out but does Trump really want the truth to come out?