Why are they being cancelled instead of rearranged?
In terms of why it can't be rearranged to another day, rugby world cup rule book states, "Where a pool match cannot be commenced on the day in which it is scheduled, it shall not be postponed to the following day, and shall be considered as cancelled."
Why that is the case in the first place I'm not sure.
Which then leaves them with the only option of rearranging it on the same day, where the reasons Adder has described come into play, i.e. they can't move 70,000 people six hours away, and that's if they had a ground big enough in the first place.
Whether they are allowed to rearrange to play indoors with no fans as the game has so much riding on it I don't know.
I suspect it's an easy decision for World Rugby to say, let's call it a 0-0 draw as then the hosts gets through to the QF which benefits most bar Scotland.
I do feel bad for Italy in all of this as they at least deserved a shot against NZ.