My cynical suspicion is the same reason that Australian infrastructure always lags behind the needs of the populace. There are no votes in infrastructure that won't be finished before the next election. Combine that with the desperate attempts to avoid the opposition labelling it "overspending", and winning the next election.
This applies to highways, railways, public transport, hospitals, power and water. etc, etc.
Maybe also, the public has learned to expect that even after all these years of practice, governments still can't write a decent contract that doesn't allow for endless cost blowouts.