Because in the 17th century armies the rank was Sergeant Major General, ranking below the General, who had overall command, and the Lieutenant General who was the General's deputy. The Sergeant Major General was the Chief of Staff/Administrator. "Sergeant" is a term derived from Latin via French "Serviens/Servant" a servant. In other words he did the hum-drum, day to day essential tasks of running the Army for the Generals.
When Oliver Cromwell organised "The New Model Army" during the English Civil War the word "Sergeant" was dropped from the rank. It was later applied in the British Army in the Napoleonic War to a Divisional Commander, Divisions being a new concept borrowed from the French. Lieutenant Generals were allocated Corps i.e. formations consisting of two or more Divisions.
Incidentally in the British Household Cavalry Regiments the rank of Sergeant is not used because of the "servant" antecedant. As all the original soldiers, including troopers, in the days of Charles 2nd, were gentlemen it was considered below their dignity to be described as "servants". The rank of Corporal,"Head of a body of men" was used. Hence "Corporal of Horse " equals Sergeant and "Corporal Major" Sergeant Major.