Relevant question asked on Offsiders on the weekend - given how long the Cricket World Cup runs for, why couldn't they reschedule the washed out Australia v Bangladesh game?
I had thought this as well but I'd imagine that the answer would be an expression of extremes combined with logistics.
The extreme angel would be well what if 5 matches were all washed out in successive days, how many days do you give them to be rescheduled as you can't say yes to one and no to another.
The logistical side is the hardest though. Accommodation will be already booked, flights as well. You're talking changing two whole teams and all their support staff so you're probably looking at about 60 or more people. You're also looking at a game where Australia were playing over 1 week away, however what if it was the NZ vs Aus game that was rained out? Then you've got to play on the Sunday with Australia having to somehow reschedule flights all the way to WA ready to go by wednesday. With so many logistical issues and also limited time between some games its just not possible as it would have to be a blanket rule across all games and that just wouldn't work.
Also I've heard people say why didn't they move it. Apart from the logistical side of things we're talking about a cricket pitch here not a field. Preparing international standard pitches requires several weeks, if not months of planning and work and to say hey can you prepare another one even a week out is a tough ask, especially when the other venue would have had to be pre-approved (aka one of the other host venues) therefore their pitches would have already been in different stages of preparation.