Maybe she didn't volunteer to testify.

If she had to be subpoenaed to appear, she'd basically be a hostile witness and Harry's barrister wouldn't be able to question her beforehand and know what her evidence was. Not knowing what is going to be said by somebody you call as your side's witness could lead to some unpleasant surprises.

She's moved on. If she wasn't interested putting her past life on display by suing the Mirror Group for monetary damages, why would she want to rake up the past to put money in Harry's and his wife's pockets?