Since they passed Obamacare on a purely partisan basis, they haven't been able to pass modifications of it since Scott Brown entered the Senate. So one answer is that they simply haven't been able to do tweaks like this legislatively. They'd have had to find a way to implement this purely within the existing law, but the Supreme Court is already critical of the law and of executive overreach. Note that Medicaid expansion was original mandatory until the Supreme Court said that the federal government lacked that authority.
Another issue is that there isn't enough money. The basic structure is that the rich and the upper end of the middle class pay for their own insurance (subsidized by a tax deduction). The lower end of the middle class receives subsidy payments for Obamacare insurance, but they still pay part of the cost. At the bottom end, the idea was that Medicaid was supposed to cover those who simply don't have money enough to contribute towards buying insurance. Medicaid's lousy coverage, but it starts with the first dollar.
The subsidy at that level simply isn't enough to allow the working poor to afford the deductible on an Obamacare bronze plan. The typical Obamacare plan is effectively catastrophic, high deductible coverage. So for most expenses, the answer is that under Obamacare, the individual pays. After that runs out is when coverage starts. But that's not the kind of coverage that the poor need. They need help with any health care expense. People living paycheck to paycheck can't afford the deductible amount. So even if the subsidy were high enough to cover the entire premium (not necessarily true), it's not enough to provide actual help.