The Posse Comitatus Act was originally enacted in 1878 to restrict the use of the United States military in many domestic matters. Originally, the act only covered the Army, which doesn't seem particularly surprising - as I understand it, the law was in reaction to events like the Great Railroad Strike of 1877, in which federal troops were used, including the US Army, whereas the Navy was not particularly involved. Therefore, it makes sense to me that the act would only focus on the Army.
However, in 1956, it was amended to include the Air Force - and no other branch of the military. Why weren't the Navy and the Marines (in particular) added along with the Air Force? They're now effectively subject to the same restrictions by dint of DoD regulations, but those regulations were only added over three decades later.