Perhaps the difference lies in the fact that the U.S. isn’t like those other countries in many more ways than just guns.
What’s the one major difference? There isn’t one. There are lots of differences including demographics, social structure and social safety nets, the relationship between citizens and their government, and so on.
But let’s not limit ourselves to gun violence. Let’s look at violence. Great Britain has a violent crime rate twice as high as the U.S. As a matter of fact, more British police are patrolling while armed than at any peacetime period in modern history.
The claim that Australia hasn’t had a mass shooting since Port Arthur is untrue, but the larger point is that more people have died in mass murders in Australia since the Port Arthur Massacre than it had in the same period of time before the event. In fact the decline in the Australian homicide rate following the gun buyback/confiscation was about the same as the decline in the U.S. homicide rate up to 2014, when the U.S. rate fell to its lowest point in more than fifty years. There has been an increase in the U.S. rate since that time, but it’s more likely due to other factors than guns.
Japan? There has been a string of mass stabbings and the country just hanged 13 individuals convicted of releasing nerve gas. The country has enacted new laws prohibiting a variety of knives (as has Great Britain).
Quit falling back on pat answers fed to you like pabulum to a baby. Learn something about what reality has to say.