ACA/ Obamacare is major medical coverage that you cannot be denied coverage for any pre-existing conditions. It is most affordable IF you receive a government subsidy on monthly premiums.If you are not getting a discount on your premiums it will be extremely expensive.
Employer plans are often great coverage for the employee themselves but the minute you add a family member (spouse or kids or both) on to the plan, that premium is going to skyrocket. This is because employers usually give benefits towards their employees but the family their employees but the family will be paying full price to be added on to the plan.
Private insurance is based on your individual and families health. There is an application process because not everyone may qualify based on health. Because you are healthy you are receiving a preferred rate so it is less expensive than most plans with better coverage.