The earliest official policy on the naming of posts and forts is found in War Department General Order Number 11, dated 8 February 1832. The order stated, “All new posts which may be hereafter established, will receive their names from the War Department, and be announced in General Orders from the Headquarters of the Army.”

At that time, both cantonment and camp designated a temporary location, while the term fort designated a permanent installation. The term post was used for any type of installation. The designation of an installation as a fort or camp was left to the discretion of division commanders

Source: http://www.history.army.mil/html/faq/base_name.html

The Army uses both names. I imagine Marine Corps bases are intended to be only temporary as they are supposed to be based off ships. So the bases would only be set up for the time period when they were not on Naval Bases or Naval Ships, but with how the Marines have been being used in the recent past the bases have become more permanent. It should be noted that the Marine Corps also calls bases depots, stations, centers and a variety of other names, depending on purpose.

Just to add on, why Air Force bases are known as bases?

Fort Polk used to be a camp before it was a fort. (This was during WWII and the Louisiana Maneuvers) In addition there is Camp Rudder which is located in Florida as part of Ranger School.

http://en.wikipedia.org/wiki/Fort_Polk

http://en.wikipedia.org/wiki/Camp_Rudder

In fact here's a list of army bases in the United States containing quite a number of camps: http://en.wikipedia.org/wiki/List_of_United_States_Army_installations

I believe your question may be flawed