The Army adopted the Mclellan because Mclellan was the General in chief of the Army.
Actually he designed the saddle around 1855/56 while a Capt of Cav. Resigned from the army around 1857. Saddle was adopted by the Army around 1859 while he was a civilian. McClellan didn't returned to the army in 1861 after the outbreak of the war, but returned with a General Officer rank instead of the Capt's rank he resigned as.