Conventional wisdom is that you have to prove you can do it on demand rather than it being a one and done. If you have one, it could be the last one. If you have two, you could have infinity.
And the first one was likely to be a dud, thereby completely removing the psychological effect from further atomic bombs, of which there was only one for the next couple months.
An interesting point here, in the Wikipedia article on the atomic bombings, that Japanese nuclear scientists reported to their government that the US could be expected to have between two and four more such bombs ready to go. They were under no illusions that the US had hundreds of these sitting around.
Not to everybody it wasn't. Some Japanese officers wanted to keep fighting even after two cities were wiped out. (Three if you count the firebombing of Tokyo, which you probably should...)
As proven earlier with Operation Ten-Go / The Battle of the East China Sea, many in the Japanese military preferred spiteful suicidal attacks to surrender, even when the hopelessness of their situation was abundantly clear.
