Becuase the rest of the world isnt the USA. Andof course we have notions out there with strict gun controls, but murder rates higher than the USA. Sure, if you cherry pick what parts of the “rest of the world” you want to compare, it can look like gun control might work. If you ignore Mexico and only look at Britain, sure.
The thing is, when gun control had & is tried in the USA there has not been a corresponding decrease in violent crime rates. Of course, excuses are made. But gun control has not worked in the USA.