I've written a lot over the past nearly three years about the damage caused to ACA policy enrollment caused by the Trump Administration's slashing of 90% of HealthCare.Gov's marketing, awareness and outreach budgets.
A significant portion of the reduction in ACA exchange enrollment in 2017, 2018 and 2019 can be blamed squarely on this.
That's not just my opinion; it's been supported by detailed analysis as well as the corresponding increase in enrollment on state-based exchanges, which operate their own marketing/outreach budgets.
The following graph compares the two over the first six Open Enrollment Periods. I've had to adjust for the fact that since 2014, several states have switched from state exchanges to the federal one or vice-versa, but even so, the contrast is dramatic and clear: