Well, at the risk of stating the obvious I imagine it is because someone wanted to cover it up. It's a human reaction to many such problems, and I don't think we need to be getting all outraged and indignant about that aspect of it.
The real problem isn't that someone hoped it wouldn't be noticed, but that it happened in the first place. The loss was the responsibility of a private company, and no doubt there'll be a fuss over it. Ministers have wasted no time in trumpeting their anger/shock/dismay etc., hoping that we'll all fall for the excuse that it was a private company's fault. Opposition speakers have also wasted no time in jumping on the condemnation bandwagon.
The truth is that the blame is entirely to be laid at the government's door - the Justice ministry is responsible for the prison service, and for the security of data within and about that service. Jack Straw may well be angry and thirsty for vengeance, but he's the Secretary of State responsible, and he's the one who should have ensured that this couldn't happen.
If there is a gold standard way of moving and keeping our data about, what is it and why is it not in universal use. Does the Government use a Gold Standard? Or does it allow departments/ministries set-up their own system?
The number of ways the data is being lost and the number of Ministries involved seems to show total chaos of data control standards