The number of insurers participating in the Obamacare marketplaces is falling. This year, 182 counties had only one insurer offering plans. Next year, that will be true of nearly 1,000 counties, or almost one-third of the total. An average marketplace will offer 17 fewer plans in this fall’s open-enrollment period than last year’s. Fewer choices make it harder for consumers to find plans that meet their needs, like including doctors and hospitals they prefer and covering the drugs they take.