Cailin, are you trying to be deliberately vexatious? Did you look for even basic information on Chorioamnionitis, as suggested by BadDay above.
The infection would have travelled through the cervix to the foetus and products of conception, which most likely were already necrotising, providing an ideal medium for the bacteria to grow and spread to the surrounding uterus and through the bloodstream, to the rest of the body.
Removing the foetus and products of conception would remove the bulk of the tissue harbouring the infection, making the infection that had spread far easier to treat with aggressive antibiotic therapy.
It would be the same if a patient had a gangrenous foot. Treating this by antibiotics alone would not be sufficient to halt the infection. The tissue of the foot is already starting to die, will never recover and become living tissue again and provide a medium for the bacteria to feed, grow and spread to the still healthy tissues surrounding it. So, you have to remove the dead and dying tissue AND treat with antibiotics.
In clinical situations like that of Savita Halappanavar, the products of conception, like a gangrenous foot, were already dying even if the foetus still showed a heartbeat. The necrotising tissues would never recover, never be able to support continued gestation of the foetus, so the foetus would not be viable. Without removing the infected, necrotising products of infection, anti-biotics would have only limited effect because the tissues cultivating the infection were still present, still generating more bacteria and enabling spread of the infection throughout the woman's body.
It seems by the time the products of conception were removed, the infection had spread and become so aggressive, causing irreparable damage to her vital organs that antibiotic and other treatments couldn't save her.
The "then she should have had a hysterectomy if the womb was also infected" argument is a complete red herring. If the infection had spread to the degree that the uterus was also necrotising, yes, a hysterectomy would have been indicated. But if the products of conception had been removed early enough, the infection would not have caught hold enough to kill off the uterine tissue, necessitating a hysterectomy.