Marines dont want dumb recruits. When I enlisted the Marine Corps and Air Force where the only branches that required a high school diploma. Army and Navy would take the uneducated at that time. The Coast Guard required a higher degree if I recall.
Also the MC would put their less intelligent troops in supply or motor pool. They claimed that they wanted the best minds on the line as bright men make better fighters.
Not sure of the merits but they did make me a 0331 machine gunner haha.
That said you have to be pretty dumb to volunteer for what we went through. It is amazing how those 4 years have impacted my life since and recommend military service to all young people.
Ooh-Rah, Semper Fi and good night Chesty Puller, where ever you are