The hospital needed redevelopment at a minimum. It's a hodge podge of old buildings with no integration. Integration is a big deal for a hospital. Nobody really fancies not having a hospital for 10 years while they build a new one. If they tackled it in stages then the timeframe would have been enormous.
A couple of people I know at the RAH right now commented that getting some patients from the ward to the ambulance to be ferried to the new RAH has actually been easier (logistically) than getting them to some corners of the current RAH. I say logistically because emotionally it's been rough. Lots of the patients were trembling and crying today. Only thing that calms lots of them down is telling them when they get to the new hospital they can see the Adelaide Oval through the windows.