Recruitment quotas are completely dependent on projected manpower needs. When the needs are low, they take only the highest qualified. College is preferred over high school and high school is preferred over GED. If they get really desperate, drop outs will be welcomed. A lot of weight is put on aptitude tests, so it's possible for almost anyone to be accepted, but when the quota can be filled with true graduates, why go to the effort of sifting through the dregs?
The numbers I saw recently showed 93% with H.S. grad or GED across all military. The ones with college are not enlisted but rather mostly ROTC officers. It didn't separate H.S. grad from GED, but I agree that GED is more accepted when needed. However, they have been desperate for more recruits for more than a decade now, and I think the the Army's official goal is to allow up to 15% GED, but since 9/11 the % has been much higher and they even started recruiting dropouts even without a GED. Air Force has the highest standards, in large part because everyone wants to fly, so they can be selective. In addition, there are plenty of kids who just barely graduate high school and have no plan to go to college because school has always been a struggle and have no good job prospects. This is who recruiters prey upon. (BTW, my brother was career military and mostly a recruiter after spinal injury in a plane jump. He told me about how recruiters would brag to each other about the bullshit they could get recruits to believe. He visited home after his first 6 months in the Army which he spent in the South and they had already transformed him into a racist and sexist who said "nigger" and "bitch" every other sentence despite never talking like that before).