Intuitively, if one were going to initiate a large program through which the federal government subsidizes health care expenses, you would want a disproportionate quantity of subsidies to go to people of modest means. People with very high incomes would get more modest subsidies. Or maybe everyone would get the same subsidy. Or you could be Mitt Romney:
To help control costs, Romney would allow all Americans to deduct from their taxable income all of their health-care costs including premiums and most out-of-pocket spending. Now, only people with a lot of expenses can deduct the cost from their taxable income.
As with all other tax deduction schemes, this has a highly regressive impact. Low-income Americans who pay FICA but not income taxes will get no help whatsoever. High-income people in the top bracket will get large tax cuts. People in the middle will get, well, a middling level of help.