According to Aetna, this decision "will affect only 49,000 of its 1.5 million policyholders in the state."
Although Aetna declined to say why it is taking this action, it's likely that the reason is California's health exchange rules. According to Reuters the California Exchange rules apply to all health insurance products sold to
individuals in the state, "whether or not they are offered through the
exchange."
Thus any company that decides after analysis that it's not worth the cost for it to participate in the California Exchange, can only avoid that cost by exiting the market - as Aetna has decided to do.