Money from the US Treasury which was helping early Obamacare is running out, and there is not enough money coming in from enrollments to sustain the program. In fact, many people who enrolled during the first year failed to continue their coverage the second year; the program was supposed to increase enrollment to stay viable.
This illustrates the death spiral of health insurance schemes which require everyone to be eligible for enrollment at the same rate without regard to pre-existing conditions. Healthy people drop out, knowing they can enroll later if they get sick. With fewer relatively healthy people paying into the system, health insurance premiums are raised to cover the high health care costs of those that carry the insurance. As the premiums go up, more people drop their insurance, because they can't afford the higher premiums. And, when their money is no longer going into the insurance pool, it is again necessary to raise the premiums.
As presently implemented, it just doesn't work, because it is not sound economics.