Which location? Inside the beltway it was always HOV on all lanes during the restricted hours from the moment it (inside the beltway) opened. That was part of the "deal" made between VA, Arlington (the VA county with the majority of the "inside the beltway" portion), and DOT to even get the road finally built.
The "single lane HOV" part was outside the beltway.
Inside the Beltway was always HOV-only during rush hour. It was originally HOV-3; but, was reduced to HOV-2. If the goal was to decrease the number of cars and increase car-pooling, they could have gone back to HOV-3.
The "single lane HOV" part was outside the beltway.