Post Office were helped by the government changing the law in 1999 so that the courts presume that evidence derived from a computer is reliable. The Law Commission that recommended this change and the courts interpreting the change have totally failed to understand the distinction between hardware (generally reliable) and software (anything but, particularly with a large, complex, bespoke system like Horizon). It is a rebuttable presumption, but it requires the defendant faced with computer evidence that they have stolen money to prove that the computer is wrong, rather than the prosecution proving that the computer is right.
To make matters worse, the courts don't like "fishing expeditions", where the defendant asks for disclosure of a lot of information in the hopes of finding something that will help their case. They expect defendants to be able to show that their request for disclosure is limited and strictly necessary. As a result the courts generally supported Post Office's refusal to disclose information that might have shown whether Horizon was working reliably, with Post Office arguing that such disclosure was costly, time consuming, wasteful and unnecessary. Defendants were therefore denied access to the evidence they needed to prove that Horizon was unreliable.
Even if the courts had ordered disclosure, it seems likely that Post Office would not have disclosed everything relevant. After all, they denied the existence of the Known Errors Log for a long time then, when they finally admitted that there was such a thing, maintained that it was simply entries such as "hit the printer here if it isn't working" and wasn't relevant to Bates vs Post Office. It was, of course, highly relevant and was one of the major pieces of evidence that helped to prove that Horizon was unreliable.