Why is April the worst month that tornadoes hit?

During the spring the lower atmosphere warms up faster than the upper atmosphere, and a warm lower atmosphere under a cold upper atmosphere is unstable as warm air is less dense and will tend to rise. Also during this time of year, even though the weather is warming up blasts of cold air regularly come through the U.S. These cold fronts combined with the unstable atmosphere can trigger strong thunderstorms. At the same time the jet stream, which remains strong at this time of year, provides plenty of wind shear to set these thunderstorms rotating. This rotation allows the storms to produce tornadoes.