For the first time in 2017, many of the nation's counties (about one-third of them) had only one insurer available on their respective Obamacare health insurance exchanges. Next year may be the first in which there are zero Obamacare insurers in some U.S. counties.

The Des Moines Register reports that after it the exit of Aetna and Wellmark Blue Cross & Blue Shield from Iowa's exchange, the only other insurer in 94 of Iowa's 99 counties is plotting its exit as well.

You might think the exit of all competitors would be a boon for the last remaining insurer, but this is not the case, thanks to two of the important Obamacare rules that are a constant subject of discussion: "Community rating," which requires that everyone of the same age be offered the same price for coverage, and "guaranteed issue," which requires that all applicants be accepted, regardless of pre-existing conditions.

Without the ability to price for health risk, and with only limited ability to charge more for age, insurers find themselves unable to make money. They could respond by raising premiums even further, but this risks making the problem worse by scaring away the healthiest customers.

The two larger Iowa insurers decided to pull out because within this framework, they were losing too much money in the exchanges. Their insurance pools attracted too many older, sicker Iowans and not enough of the younger, healthier ones who make the pools financially stable. Medica fears that its competitors' most motivated insurance customers — the sickest ones — are going to flood their small pool, creating the potential for massive losses.

If insurers completely clear the field in most of Iowa, then the only people with coverage will be those with grandfathered plans, employer plans, or Medicaid. Note that Iowa did expand Medicaid, so this is not a state where Democrats can use that excuse to complain about Republicans sabotaging the law — it's just failing.