I think the defence thought the prosecution didn't have one clincher piece of evidence, such as direct proof she picked up insulin and injected it, or CCTV, or a death certificate which said non-natural causes, or anything, so they did not go in hell for leather to disprove each individual tiny piece of the jigsaw puzzle on which she was eventually convicted. That was a mistake, because as we are now seeing, much of that evidence could at least be probed for reasonable doubt.
I don't even believe she's innocent! I don't like to see this type of evidence thought to be evidence though- they didn't run the right tests on the insulin to establish it was endogenous or exogenous and so now no-one will know. What this whole case shows is that babies in NICU are very very vulnerable, and so extra precautions (e.g. CCTV which everyone told me was never going to be possible) and proper testing when babies die unexpectedly needs to be done more thoroughly. This did not happen, and other places in the hospital also had spikes of unexpected deaths where Letby wasn't working, so it seems it was a highly unsafe environment, whether or not they additionally had a baby killer in their midst. In fact, the fact they had an unsafe place and weren't equipped to treat those babies properly has contributed to additional deaths full stop even outside the ones she's accused of, and if you were a killer, this would be the perfect place to work.
High mortality amongst babies is appalling, and not just 'one of those things' whether it be to unsafe and unsanitary environments, or to malicious individuals. This case has highlighted this, I wouldn't have wanted my baby to be treated either by her, or in this environment.