It can seem random now, but at times of war new military units are created and numbered in sequence. After the war the military downsizes so it “demobilize” certain units and distributes their troops into other more established units. If a unit distinguishes itself through some kind of specialization or combat achievements then the leadership keeps it around as the skill and legacy can both be powerful tools. So the 101st got kept because it was a proven combat unit with airborne training. While other less experienced or less trained units were demobilized after the war and their troops distributed to other units.
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