Because there is nothing other than circumstantial evidence, flawed evidence and assumption of guilt. There is no forensics, and no eye witness.
Our legal system is not perfect, but you are not supposed to be found guilty of crimes if there is a reasonable doubt over your guilt - that may mean the odd guilty party goes free, but that is the penalty you pay for preventing, by in large, the innocent from going to prison.
Her defence were clearly incompetent in missing many of the issues that have been picked up by interested parties (often qualified in medicine) taking a cursory look.
She could be guilty, in which case in prison she must stay, but I for one think the whole sorry situation needs looking at again as miscarriages do occur - take R vs Evans, they were sure he was guilty as were the public, and hung him, but he was, in fact tragically innocent.