The carpool lanes were very under utilized. Hybrids and later EVs were also slow to be adopted, and the state wanted this adoption accelerated due to air quality and just general environmental consciousness.
So the state decided to add the carpool benefit, which solved two problems.
Now that EVs are far more abundant, that policy is getting revisited. Which is fair, because the carpool lane can only support so many before it just gets clogged like the main road. And people don’t necessarily need the encouragement to get EVs anymore.