Because the more cases there are in one day, the more people that go to hospital in a short period, when the hospitals are already full and short staffed.
Better to have 2 million with it over 3 weeks than a few days, simply because it it gives people a chance to enter and leave hospital over a period than all at once as stays are shorter it seems. Seems very simple to me.
Just going for it is a gamble might be alright, then again it might not, doesn’t seem a gamble worth taking to me, maybe ask the hospitals themselves, for the sake of a few weeks of fairly minuscule restrictions.