How do you predict the benefits?
The problem with doing a cost-benefit analysis of major infrastructure projects is that their very existence transforms the environment around them. We have all seen entire towns spring up when a new freeway is built - which usually results in even more crowded roads that the freeway was designed to fix.
A project like the NBN has the potential to completely transform almost every aspect of daily life with enormous benefits for transport, housing, energy use, education, health etc, etc. Unfortunately for anyone attempting this analysis it would be impossible to put a value on those benefits, even if we could predict which ones will eventuate and what form they will take.