Because of two reasons:
1. A shortage of primary care providers in particular locations
2. Even with the ACA and/or expansion of Medicaid, not a lot of providers will accept that insurance because of lower reimbursement rates.
The safety net clinics address both issues. The alternative is to close those facilities, have the patients go to the ER, and have the hospitals incur the bad debt. The problem with that is the hospitals do not have an unlimited capacity to see these patients or absorb the bad debt and end up in financial distress.