Why doesn't Florida have more violent tornadoes?

The strength of a tornado depends on the strength of the thunderstorm that produces it. The strength of a thunderstorm is affected by instability, which in simple terms involves cool air over a warm lower atmosphere. Florida is a semi-tropical area where the upper atmosphere is warmer than in other regions, leading to less instability an thunderstorms that are generally weaker than their counterparts in Tornado Alley.