We thought that if we didn't do that we'd run a very real risk of the entire healthcare system collapsing. We were trying to "flatten the curve" so that hospitals could deal with an ongoing roar instead of being hit with a massive explosion.
Not were, are.
The entire lockdown/restriction/mask mandate set of policies has been based on managing hospital capacity. State and local governments are still using hospital capacity to decide when to lift or reimpose restrictions.
The wave of explainers we got last year about flattening the curve were wrong about how long it would take, but the fundamental strategy and the reasoning behind it never actually changed.