Without researching it, I expect that the reason the Army decided against HE and the Marines for it was the different anticipated Armor missions. The Army expects to mainly (perhaps even solely) to use it's armor to fight opposing armor, making basic HE rounds a useless item. The Marines - on the other hand - expect to fight infantry in dug-in but not actually armored positions, making HE rounds an essential item. 

Of course, they both might be wrong, particularly the Army. I remember reading a book called "Tank Sergeant," written by a guy who crewed an M-60 tank in Vietnam. He had spent his entire career prior to that point learning to fight Soviet T-72s at 1000 yards in Europe . . . only to have his entire combat experience turn out to be against lightly-armed infantry fighting from jungle revetments, which were generally encountered at touching distance.