Joker has put joker serum in the water, scarecrow put fear toxin in the water, i cant remember for sure but i think there is either a lazarus pit under gotham or an actual demonic entity.

In addition, after harvey got dented, gothams last hope became batman. And hes not really the rehabilitation and reintegration type, moreso the “beat you so bad you cant afford your medical bills even in a fictional comic” type