Why did Charlemagnes empire dissolve after his death?

It did not really dissolve. It was - in complete accordance with the Frankish custom of the time - divided between his three sons. The western part was more or less what was to become France; the eastern part covered most of what was to become the Holy Roman Empire. Only the third part (in the middle) more or less 'dissolved' over time into a number of Principalities that linked themselves to either France or to the Holy Roman Empire.