There are a number of reasons Ramirez in particular was so popular. As serial killers go, he had a youthful, dark handsomeness to him. More importantly, though, he had an incredibly fucked up backstory that would make you sympathetic to him if he had ended up as almost any kind of criminal but a sexually sadistic serial killer. He suffered serious head trauma twice and probably had brain damage from that, was routinely beaten by his father, and had a psychotic cousin (more like an uncle) who showed him photos of his rape victims and even a beheaded woman he had taken of himself in Vietnam. This same charming dude also shot his wife in the face in front of him when he was 13 or so.
So you have the good looks, the tragic backstory, and on top of that, the sexually sadistic violence that seems to really turn these freaky chicks on. Whether they wanted to be his victims or to be his female savior (or both), he more or less pushed every button that could be pushed for this kind of person.