Duration, duration, duration
Typical ministerial tenure: about 2 years.
Typical large IT project duration: 6-10 years.
In other words, the person responsible for starting the project has zero expectation of seeing it through to the end. And when she's replaced, her successor will want to make his own "improvements".
The contractors, of course, know this, and they know it makes the contract basically a blank cheque. That's why they'll fight like demons to win it.
Incidentally, the same objection applies to yesterday's Tory brainwave of outsourcing back-office IT functions. Whenever you create new channels to give public money to private companies, you get new avenues for abuse and corruption.
Just stop it. If the data won't fit in a spreadsheet, the gov't has no business collecting it anyway. So let's run all public services using nothing but Excel. We'll save a fortune in the short run, and an even bigger fortune in the long run.