The judge prevented her from calling expert witnesses case by case, allowing the prosecution expert witnesses a long monopoly at the beginning of the trial. In cross examination, the chief expert witness for the prosecution changed his story about what had happened, frequently. The initial defence reports had been written to counter the prosecution reports, and the prosecution expert witness saw them before taking the stand (and in some cases changing their story).
The question Letby's defence had to ask was, had they discredited the prosecution expert witnesses enough in cross examination that they could avoid calling their own experts, whose reports were written to respond to the accusations, not to explain the deaths.
(It wasn't their job to explain the deaths, because you never have to prove innocence, only guilt. And they were prevented from using the external reports Chester commissioned which found poor conditions and failings in care as contributory factors).
We don't know why Letby didn't call her experts in the end, but we know the defence wanted to use them early in the trial and that the chief among them, Michael Hall, has raised concerns about the safety of the conviction since. So the explanation isn't that the reports said Letby was guilty.
Most likely it was a tactical decision, unlikely to have been Letby's suggestion but legal advice - and legal experts haven't found it surprising or incriminating.
Here is a useful article on why Letby may not have called these witnesses, for anyone who is interested
davidallengreen.com/2024/07/the-lucy-letby-case-some-thoughts-and-observations-what-should-happen-when-a-defence-does-not-put-in-their-own-expert-evidence-for-good-reason-or-bad/