Health-insurance customers in a growing number of mostly rural regions will have just one insurer’s plans to choose from on the Affordable Care Act’s exchanges next year, as some companies pull out of unprofitable markets. The entire states of Alaska and Alabama are expected to have only one insurer on the health law’s signature online marketplaces next year, according to state regulators. The same is expected to be true in parts of several other states, including Kentucky, Tennessee, Mississippi, Arizona and Oklahoma, state regulators said. So far, more than 650 counties appear on track to have just one insurer on...