On second thought, an even better possibility relates to the fact that Alexandria was very concerned about the traffic impact of the opening of the Mark Center on local streets in the West End (e.g. Seminary, Beauregard, etc). Many jobs were moved from the Pentagon, which has the slug and transit commute options, to the massive building at Seminary and 395.
The retention of the HOV-3 only rule, even after the corridor was converted to HOT, may have been in order to discourage single-car commuting to the Mark Center.