When the records started being kept in the mid 20th century, there were far fewer people and far fewer people to report the tornadoes.
The Doppler radar can detect very small tornadoes that would normally be overlooked. There was a time when the authorities didn't have a way of determining the force of tornadoes when nobody was a witness to the actual tornado. All of these factors increase significantly the number of reported tornadoes.
Doppler radar was deployed in the early 1990s. The graph at the end of the article indicates instead the number of reported tornadoes rather than the number of tornadoes.
There is a tornado scale starting with EF: The higher the number, the more intense the tornado. In particular, EF0 is the lowest with three second gusts of between 104km/h to 136 km/h. A tornado of EF5 is the highest with three second gusts exceeding 320 km/h.
The first graph shows everything with gusts over 136 km/h from 1954 to 2014.
This other graph shows F3+ tornadoes also from 1954 to 2014 and we see a drop in the most intense tornadoes in the United States.
The last graph here shows number of reported tornadoes which include tornadoes after the early 1990s which were excluded because before that they didn't have Doppler radar and several other factors.
Source: https://www.weather.gov/lsx/enhancedfujitascale
Source: National Oceanic and Atmospheric Administration U.S.A.*