When Jack the Ripper was on his murder spree, there were an estimated 1200 prostitutes in the Whitechapel area. The city of London had about 80,000 prostitutes total.
As dangerous as it was, prostitution paid better wages than any of the other jobs working-class women could get in the city, such as being a scrubmaid, sewing in a sweatshop or making matchboxes. Those who slaved at regular "women's work" could at most expect to get about ten pence per 17-hour day of hard labor. Prostitutes, on the other hand, earned two or three pence per john serviced.
Jack's murders were less terrifying than the specter of starvation to most working women, so they continued their trade as best they could.
Reference: Murder Ink edited by Dilys Winn (Workman Publishing Company)