To answer your question, you have to realize that most working Americans get their insurance through their employer. And employers leverage the size of their labor pool to negotiate favorable insurance plans on their behalf.
The nature of that dynamic means that the smaller the employer, the less leverage available for insurance negotiation. Which means more expensive plans. In many cases where incomes are modest, this means that small employers and the self-employed are effectively priced out of the health insurance marketplace because insurers have no incentive to craft affordable plans that also meet most of someone's healthcare needs.
Obamacare essentially aimed to address that group of people by combining them into a single labor pool, like you would find at a large corporation, and thus leveraging that scale to eventually reduce the cost of insuring them. That's the group it's aimed at and, yes, there are millions of such people.