Navy fills billets for enlisted female submariners

The Navy says it has filled its first group of billets for enlisted female submariners.
Navy surveying enlisted women about submarine force
http://www.stripes.com/news/us/navy-fills-billets-for-enlisted-female-submariners-1.353962

WHY NOT HAVE ALL-FEMALE CREWS AND ALL-FEMALE MILITARY UNITS IN ALL THE SERVICES THE WAY THE WACS,etc. USED TO BE?