Capita may preside over a succession of shit-storm projects, but the truth is that Capita frequently deliver in the private sector projects they undertake.
So, why does it go wrong with public projects? No ownership is my bet. Every elected person involved in the project will think that they are the boss, and continually add minor tweaks and changes to the specs. Civil servants ignore the politicos mainly, and will supply their own requirements.
Add enough of these people to any project, and you'll very quickly have scope creep and fail to make any deadline.
It's all cool to rag on Capita and their ilk (lol, "Crapita"), but the contractor actually working on this project is likely underpaid*, couldn't give a fuck whether the requirements are sane or germane, and so just does as he is told. After all, if it goes wrong, they can show the broken requirements, say "not us guv", and do another £50m in fixup work.
Successful projects usually have strong product ownership - someone who knows exactly where to take this thing, and has final say over everything. Unsuccessful projects rarely have strong ownership.
* Yes - even though the company makes a mint out of government work, the contractors they are supplying are probably not - well, not compared to typical consultants.