Long term government contracts
There's inherently several issues that won't help - and I'm in no way defending the process here:
1.) Projects will almost certainly change scope during their lifetime (political masters being what they are and all that). A bit like Triggers broom, it'll bear little resemblance to the original specification after a few years so it's hardly surprising the software - all of which have 'snag lists' that require re-working during roll-outs - struggles to evolve correctly;
2.) The project leaders change over time. This makes it far more difficult to hold people to account, especially if they've long retired;
3.) Government ministers are not project specialists: they outsource this job to those that are. I'd argue that, pretty much by definition, someone very specialist in designing the spec. for a system such as Horizon must come from a supplier that has skills in that area. As such, their project definition can only be skewed in favour that supplier's solutions even if it's not the ideal fit.
Add in the inevitable 'incentives' and blame dodging ('Yes Minister' and 'The Thick Of It' are not comedies, they're documentaries) and, well, it doesn't exactly aid transparency and critical thinking!