I get the feeling that it's nothing to do with being a Government agency. I've seen more than 50% failure rate on very large IT projects for other regular businesses and corporations.
There seems to be a major problem with sotware projects producing an accurate requirements spec, and following that though to implementation. End users have no idea what they want, fill the requirements full of edge cases, and keep moving the goalposts. Programmers often have no idea how the software will be used so whenever there are gaps they improvise with the most ridiculous schemes. And software architects always say "technology XXXX will save us, it makes YYY so easy", forgetting entirely that you still have to produce a sensible user interface with a sane workflow and that takes 80% of the effort.
Personally I cant see this getting better for a while. It's not the fault of any one person, it's just human nature when trying to deal with highly complex systems. We need to use a radically different design approach and employ exceptionally good project managers, and even then we might still want to cross our fingers.