Why does the us have the most tornadoes each year?

There are two reasons. First is that the United States has an ideal climate setup in the middle part of the country. In the spring storm systems cause warm, moist air from the Gulf of Mexico to collide with cool air from Canada and dry air from the Rockies, resulting in the formation of strong thunderstorms. About a mile off the ground a layer of stable air holds thunderstorm activity back, allowing instability to build underneath it. As a result when the storm break through the cap the instability will be enormous, leading to explosively powerful thunderstorms. At the same time wind speed and direction varies with altitude, which sets these storms rotating and also strengthens the updraft. This rotation combined with strong thunderstorm updrafts can then produce tornadoes.

The second reason is that the United States is very good at keeping record of the tornadoes it gets. Many other countries that have a high incidence of tornadoes do not keep detailed records, and many tornadoes are never recorded at all.