Cloud-to-ground lightning in Florida?
Your map of lighting tells you where thunderstorms occur.  Your second map of watches tells you where a subset of thunderstorms exhibiting severe threats (high winds, large hail or tornadoes).  The type of thunderstorms that frequent Florida are air-mass thunderstorms and generally will not warrant a severe thunderstorm watch.  The storms in the midwest that do get watches are typically of the supercell variety or associated with squall lines or bow echoes.  

Your thesis that "The midwest clearly has the highest concentration of thunderstorms" is incorrect.  What your data says is that the midwest has the highest concentration of thunderstorms producing severe weather threats.  This is not the same as concentration of thunderstorms.  All thunderstorms produce lightning (by definition), not all of them are severe.

Convection in Florida is driven by the sea breeze and afternoon storms happen there like clockwork.  These storms produce lightning but no threats that would warrant a severe thunderstorm watch.  These storms last about 45-60 minutes and produce lightning.  This also makes Florida the lightning capital of the US, as the data in your question shows.