HOV lanes are nominally intended to encourage more ecological use of transit resources, with congestion relief as a coincident effect (fewer cars on the road -> less pollution, lower energy usage, reduced rate of needing new transit infrastructure).

Since electric cars serve many / most of the same purposes, it makes sense to append them to a reward system which has already proven effective at driving adoption and retention of preferred behaviors.

If it's spare HOV capacity, then I agree. But if adding the electric cars to HOV lanes increases the congestion in HOV lanes, it might degrade their effectiveness in incentivizing the behaviors they currently incentivize. For example, the attractiveness of either carpooling or riding the bus declines if HOV lanes get slower. Carpooling in particular needs enough of a delta in the regular v. HOV trip times to make up for having to pick up the other carpool members (unless you live in the same place).

One solution is to allow a capped number in, to incentivize purchase but only up to a threshold that doesn't overly degrade HOV trip times. I believe that's what California did for hybrids, allowing HOV usage for the first N purchased.

NorCal still has a couple things that they can do to alleviate congestion in carpool lanes. They can extend the hours to the carpool lane, or even make it HoV for 24 hours a day (SoCal does this).

My guess is that in a couple years we will see 24 hour carpool lanes in NorCal.