A blockade would mean a slow death for thousands or potentially millions of Japanese, as well at the cost and risk of tens of thousands of American troops, as well as a stupid amount of resources and infrastructure.

The message in Hiroshima and Nagasaki was "We can put eleven men on a fancy plane and make one of your main cities simply disappear, and you can do nothing about it."

That made not surrendering simply and objectively not worth it.