That is because we have two very broad oceans on either side of us and two very weak nations to the North an the South. We are very well insulated from direct invasion.
The Soviets and Israelis are not that fortunate. They had to employ women in large numbers to maintain their very existence. I suspect that should we ever have to face that dire a circumstance a lot of traditional male only military positions would fall by the wayside.
We are also not a totalitarian Communist state like Stalin's Soviet Union--at least not yet.