Prime Healthcare Services Inc.'s proposed deal to buy six financially struggling Roman Catholic hospitals has collapsed, raising questions about how the nonprofit chain will keep its doors open. The Ontario-based chain of 30 hospitals nationwide decided against the $843-million purchase of the hospitals, which include St. Vincent Medical Center and St. Francis Medical Center in L.A. County, according to a memo to employees from the hospitals' chief executive. Robert Issai, chief executive of Daughters of Charity Health System, wrote that Prime had found the conditions imposed by state Atty. Gen. Kamala Harris on operating the hospitals too onerous.