Why did scientists not know the 1989 San Francisco earthquake was coming?

It's the simplest reason really, because the earthquakes can not be predicted. Talking about a short term prediction, yes, some occurrences give heads up that an earthquake is coming, though it is never a sure thing, but speaking long term, no earthquake can be predicted. Scientists know which are the places where earthquakes will most likely appear, but very often they are up to a surprise.