Possibly because she was advised to keep her mouth shut: she might be averse to opening it. Anything she said would be scrutinised (for example if further info has come to light that doesn't support her version of whatever she chose to discuss). Therefore she would have to be very careful about what she chose to discuss.
I don't see that why she would be advised to keep her mouth shut.
Her testimony is her word against Jeremy's. So can't really be proved wrong.
She could just expand on what she said in her WS -
What was his voice like when he expressed his hatred and plans.
Why didn't she believe he would do it.
Was it a cough or a laugh Stan Jones heard.
Did he show any remorse afterwards.
Whose idea was it to rob the caravan site.
Did she have no choice but to go around with Bamber for a month afterwards.
Was she aware Bamber was a suspect when she approached the police.
Why did she wait a month before going to the police.
Why did she cry at trial.
Why did she accept the NOTW offer.
All would make interesting reading. And enhance the book. It wouldn't put Julie in a bad light either.