Before there was tonnes of data about all of us all over the place.
Why not link together:
- HMRC systems
- Local government systems
- GP/hospital records
- School systems
Then they would have all the data they're asking for in the census and more.
Because you'd end up with duplicated data, out dated data, and missing data.
The whole idea of a census is that it gives an accurate snapshot of the population at a specific time so you shouldn't be getting Mr Smith counted twice because two different systems have him down slightly differently, or you end up having to try and work out which data from which system is actually correct at the specific time which is a mammoth task.
Also a lot of the other systems have highly sensitive information on them, so you ideally never link them together more than you absolutely need to for data protection reasons...
To give an idea, my brother in law found he wasn't registered at his GP, apparently he'd not been for long enough they'd removed him, whilst there are accounts of people who've died still being on a GP's register years later because the GP hadn't been informed.
HRMC is usually working at least a month, sometimes up to a year out of date, the hospital records don't necessarily show where you are living at the moment or at all if you've not needed a hospital visit for a number of years so the hospital and your GP might have different records neither of which may be correct for where you actually are.
It's also worth remembering that many government systems don't ask where you are living, just where you can be contacted because people like students might be living at one end of the country but still going "home" to the parents over the holidays.
The Census gets all sorts of information together in a fairly cheap way, probably far cheaper than linking the other systems, let alone linking them securely and then working out what data is accurate when you've got conflicting information, and gets information you don't necessarily get on the other systems - I'm not sure which government system would actually know everyone's specific job, HMRC might know your income but that doesn't help with things like working out if certain jobs are increasing, decreasing, disappeared or are new (and if need be work out if support is needed to keep skills alive).