Mainly because its so contagious and everyone has been caught out underestimating it. It isn't just America though, it's everywhere.
Japan is doing well, mainly because they're used to wearing face masks and have supply logistics already in place. Don't be fooled though! We're always being told masks aren't effective even though they catch debris from coughs and sneezes and stop face touching. At the same time we're also told one of the main ways to catch it is by touching our faces. The Japanese also have good standards of cleanliness in their public places and transport systems. Perhaps there's a lesson to be learnt here!
It's also worth mentioning that when it started no one had any natural immunity because it was a virus new to humans. That means everyone can catch it, and everyone who has it can pass it on.
Isolation measures aren't designed to stop you catching it, just to reduce the number of infected people at any one time so, if you're unlucky enough to need it, there may be a bed and ventilator available in the ICU.
The hope is to slow the rate of spread until a vaccine is available to reduce the host supply for the virus
It's what makes this a crisis. How rapidly it can spread. It is like that stomach flu that gets on cruise ships. Just touching something that another person who is infected has touched will likely cause a transmission. Its on your hands and it lives on whatever you put your hands on for up to 24 hrs. Anyone at the post office could potentially spread to this to thousands.