It's not unusual at all to not testify on one's own behalf.
Most lawyers recommend against it because it goes sideways so often. People often crumble under cross examination and make themselves look bad.
She probably did the right thing. She's not a sympathetic figure and being one is the only reason to testify; you want the jury to feel sympathy.
She's a snotty, privileged woman who ran around with a man who is now disgraced, hiding in the woods of New Hampshire in a mansion until she was caught.
There's nothing she can say to make the jury like her.