There was a deliberate press* blackout on those Japanese balloons. Several of them did make it to the US but they didn't do a lot of damage except for that one that killed 6 people. The reports were blacked out because we didn't want the Japanese to know that any of the balloons were making it across the Pacific. If they knew they were having some successes, they'd be more likely to send more of them across, perhaps armed with more dangerous substances than incendary devices. Do a Google search on Unit 731 to get an idea of what they could've sent.

* Imagine, there was a time not so long ago when the Press actually cooperated with the military when it was in the national interest. I doubt I'll ever see such a thing in my lifetime.