Because it may end up costing more in the long run. This year is basically a wipe out for films. Right now if you're a smaller film scheduled for release Netflix is looking like a saviour because they are likely to buy and distribute your movie meaning more people have a chance to now see it. But these big tentpoles run massive marketing campaigns in the hundreds of millions in an attempt to get people to watch it, which they will now have to do all over again. As I said though, contracts are probably in place which prevent this, but it would surprise me if there was no natural disaster type of clause.