The culmination of the rise of the fiscal military state. If you look at the 18th century in general you will see that compared to the 17th century there are almost no active heads of state in military affairs with notable exceptions such as Peter the Great of Russia and Frederick the Great of Prussia. This was due to the ever increasing financial burden on states when undertaking military action due to the military revolution that had led to an increase in the number of troops as well as specialization and training of these troops and the formation of standing armies all of which, when combined together, dramatically increased the cost of war. Moreover as the theatres of war increase in scale (war began to take place on multiple fronts, often on separate continents), it becomes impractical for a head of state to be present in all military affairs. Effectively the head of state's main role goes from being on the battlefield to working domestically in conjunction with the state in order to finance the war effort. Napoleon himself was an anomaly not the thumb of rule in terms of Napoleon doubling as head of state and battlefield commander.

Edit: Corrected an error pointed out below