Brokers began leaving the marketplace because many insurers stopped offering them commissions on ACA plans.

It started when the three of the largest insurance companies — Aetna, Anthem and Cigna — announced in the beginning of 2016 they would no longer pay commissions for customers who signed up through the "special enrollments" provision of the ACA, which they claimed was being exploited by customers who were forgoing insurance until they needed treatment.

The Centers for Medicare and Medicaid Services tried to reduce abuse by introducing stricter rules for special enrollment periods.

Nevertheless, over the course of last year insurers became even stingier, cutting or doing away with commissions for brokers who direct business to their ACA plans. Some insurers began to sharply reduce commissions altogether, arguing that it would help them reduce premiums. Blue Cross Blue Shield of Illinois stopped offering commissions to brokers for individual policies — both ACA and non-ACA plans — a year ago.

They have put particular emphasis on discouraging enrollment in high-level "platinum" or "gold" plans, a practice that in some instances likely runs afoul of the health law, which requires insurers to pay commissions for ACA plans if they are paying them for similar plans outside of the marketplace.

Nevertheless, the practice appears to have had the intended effect. Modern Healthcare reports the number of customers opting for gold plans is down by nearly half (47 percent) from two years ago, while the number picking platinum plans is down 90 percent. Meanwhile, the lowest level bronze plans are up 6 percent.

While insurers cut commissions to discourage business they said was undermining their health plans, there are now concerns that the absence of brokers in the ACA business will only exacerbate the marketplace's problems attracting young, healthy customers, since roughly half of those who sign up for an ACA plan do so through a broker.