That's explained by columnist Michael Hiltzik in Trump's own figures show that Obamacare is working well for the vast majority of enrollees:
President Trump brags that he has “gutted” the Affordable Care Act, but statistics released this week by his own Department of Health and Human Services show that it’s holding up well against his onslaught, with nearly 90% of all enrollees experiencing affordable, stable or even declining premiums.
Middle-class consumers who make too much money to obtain ACA subsidies are being hurt, however, in the face of shrinking competition and higher premiums. Hiltzik's article discusses proposals to extend the subsidies to help more people pay premiums.